There are still many publishers that come out with a lot of adventure support for their games. WotC doesn't, but that's just been a pet problem of theirs for a long time. They believed it did not pay off for them (maybe it didn't because of their large corporate overhead) and they spread that idea through the industry. And of course little guys barely get a corebook out, it's not like they are publishing a whole line of material and just not adventures for their games.

I myself prefer well-supported game lines. I play and GM a lot, and am a busy professional and dad, so though I do make up some of my own stuff, I certainly appreciate a lot of published adventures to put into the mix. Therefore we have:

Paizo. I won't mention the king of adventures since another answer has, but they are clearly the acme of the idea of adventure products (they are putting out more, and better, adventures than any company ever in RPGs). But wait, I said I wasn't mentioning them.

Chamado de Cthulhu. Their game line is built solely on the strength of the adventures; splatbooks are almost nonexistent, then there's some setting books, t hen it's all adventures. There's a whole bookcase shelf with nothing but CoC adventures in my library, 50+ of them with many being quite large, and they publish more regularly. Some are old and reprinted, some are new, but you can get enough to strangle a horse with. They also let others publish - there's the classic Pagan Press stuff but I got some small "module style" CoC adventures just recently from Super Genuis Games.

Muito do que Mongoose Publishing does has good adventure support historically; Paranoia is lively right now for example; I'm not sure which of their lines are active or not but they put out a lot of support for Conan, Traveller, all their weird 2000AD stuff, etc.

Pelgrane Press does well too with adventures for their GUMSHOE-based games: Trail of Cthulhu has a whole bunch but there's also a couple good adventure collections for Mutant City Blues and Esoterrorists. Their new SF one, Ashen Stars, is shipping an adventure collection as the first thing out of the gate.

Mundos Selvagens focuses a lot more on adventures than splatbooks. Some from their actual publishers, but lots and lots from third party publishers. At my FLGS there are a bunch of their larger campaign books and some small "4 linked adventures" little chapbooks.

tb Mutantes e Masterminds has a lot of adventure support from Green Ronin (here's a dozen freebies) and third parties through their M&M Superlink program.

Here's a suggestion - go to RPGNow and drill down on "Adventures" and then open up "By Rule System" and you can easily see which game lines are adventure-heavy. (For example, it lists 174 adventures for Savage Worlds!) No dearth of adventures here.

Seems to me that "there's no more adventures" is largely disinformation spread by WotC (as if any corporate economic advice that applies to them, a high staff and overhead part of a huge conglomerate, is relevant to any other RPG company) and seemingly confirmed by all the little bitty players that never get more than 1 or 2 books out for a whole game line anyway (Here's my corebook! Here's me going out of business or getting distracted by something else shiny!). But if you go down the list of other major publishers, you see large and healthy adventure support.

Most companies have gotten the hint that they need to have others producing adventures too - besides the OGLed games like d20 and FATE, most of these publishers have some kind of "you can publish for us easily" agreement (like CoC, SW, M&M...), meaning there's a lot of third party products and free fan adventures.

To look at why Adventures are less prominent now

• Adventures are a hard item to write well... good adventures:
  • have to be suitable for a wide range of characters
  • have to have alternate paths in case of player failure or diversion
  • have to be written exceptionally clearly
  • need targeted art done specific for the adventure (stock art won't do for maps...)
• Adventures rely upon a style of play that is generally very GM-focused.
• Adventures rely upon the GM being able to either coerce, convince, or lure the players into them.
• Adventures have to be set somewhere
  • many games support multiple settings
  • most games settings don't allow for cross-over adventures to work easily
• Adventures draw fewer customers than player supplements
  • lack of replayability often reduces sales
  • difficulty of adapting to alternate settings reduces sales
  • GM only purchase reduces sales potential drastically
    • many GM's don't buy them anyway
  • system mechanical differences reduce salability - it's not good enough anymore to be a generic fantasy adventure.

Who Makes them anymore?

Several companies, including Paizo, AEG, Dungeon Crawl Classics, Mongoose Press, Chaosium, Fantasy Flight Games, and even Wizards of the Coast, produce adventure modules for their game lines.

Mongoose produces adventures as additional content for most of their Traveller line, and includes them in the supplement itself. A few stand alones have been released commercially, and several as free content. They are not great adventures, but are useable. 1001 Patrons is not an adventure, per se, but a collection of 1001 adventure seeds with multiple outcomes to prevent player foreknowledge.

Paizo produces well respected Pathfinder modules, but given that Pathfinder is often described (including by Paizo) as "D&D 3.75", that won't suit your needs.

Fantasy Flight produces several game lines. WFRP 2E had plenty of adventures, and WFRP 3E has several; they are not cross compatible, but a fan effort to convert 2E and 1E adventures to 3E is underway. the 40K series has excellent adventures; again, not suitable for you. There should be excellent support for their forthcoming Star Wars RPG, but it's not expected until 2012.

Dungeon Crawl Classics are for D&D 3.5; they are not suitable to your needs as expressed.

AEG's Legend of the 5 Rings game has a number of published adventures for older editions; these are perfectly usable with newer editions by bacdating the campaign. Further, every supplement seems to have several semi-finished adventures in it, most of which provide a "hook, line, and sinker" allowing the GM to pick and choose. Several older supplements also include a full on 10-page or so adventure in addition to the adventure seeds.

Chaosium has several adventure modules for the various BRP settings, most especially Call of Cthulhu. Since I don't play BRP anymore (and rarely did in days gone by), I can't speak to their current offerings. I will say that the old ElfQuest modules were (and remain) excellent.

Wizards' adventures require DDI subscription to access, and don't suit your needs.

SJG produces few adventures, but the few they do are quite well done. Many can be converted.

Flying Buffalo's Túneis e Trolls has a HUGE back catalog still available, and more make it to PDF each month. Most, however, are solitair play, and thus not suitable for your needs.

Legends of the Ancient World likewise produces almost exclusively Solo modules.

Older stuff to look at

WFRP 1E had two campaign series: The Enemy Within campaign and the Doomstones campaign. Doomstones looks like a converted D&D adventure, but works well. With a little effort, it can be converted to other systems easily. TEW, however, ties tightly to the WFRP setting, and is also a series of sourcebooks; each module is 1/4 sourcebook, 3/4 adventures. It can take over a year to run TEW, and several months to run Doomstones.

Classic Traveller has many excellent adventures. Even as I dislike the lack of mechanics in CT, I've run most of the CT modules using later Traveller editions with little trouble. MegaTraveller and Mongoose Traveller both can run CT adventures with only trivial changes. Better still, you can buy all of them on a $35 cd. More good Traveller adventures are on the JTAS CD, also $35. Be warned: many CT adventures require GM's to fill in details.

Twilight 2000 1st edition has a raft of good adventures; I've run several of them. Each also serves as a sourcebook; most are about 1/5 to 1/4 sourcebook. It's also available, complete, on a $35 CD in PDF. Every GDW produced bit. 2E has less well done adventures, IMO, but it's also available as a complete system CD for $35.

FASA's Star Trek had a lot of really good adventures. Most can be adapted to other Star Trek systems pretty easily, but one needs to know how FASA set up the "big map" for a few of them. (They didn't use the SFTM map as a base.)

Last Unicorn Games' Star Trek lines had several good adventures and a decent campaign. While less easily ported than FASA's, they're still fairly easily ported.

I think it's worth highlighting the dramatically changed D&D landscape.

Parts of 5th Edition of D&D have been open-sourced, and it's available as a free to download 400-page PDF!

• WotC under 5E is now printing "big adventure" book every 6 12 meses.
• They've also opened up +50100+ Adventurer's League adventures for a couple of bucks each. All available on DMs Guild.
• A search on DM's Guild for just 5th Edition Adventures brings up hundreds of them, most available for a few dollars.
• Wizards has made much of their back catalog available for PDF download on the DM's Guild sister sites (DriveThruRpg RpgNow). If you want to find adventures for Dark Sun or Planescape, those things are available for download. If you want to run them in 5e, there are several well-organized adaptations available for free from the community.
• And thanks to DM's Guild licensing, the third party adventures can now happen inside of the existing worlds. So here someone assembled a 5-star 80 page supplement for Ravenloft.

One of the community members, Merric Blackman, has begun assembling a master list of all of the adventures available for download. There are hundreds here, many with reviews, dozens of them are free.

And that's just D&D 5E, the Paizo / Pathfinder people now have hundreds of Adventures & Modules. They even sell subscription packs and their Pathfinder Society organized play is fueling the on-going creation of Adventures. People are now Kickstarting adventures. Somebody has written an entire campaign and multiple adventures set in a "My Little Pony" world... it has oodles of 4 & 5 stars reviews! Another group is building RPG adventures for kids.

And most of the game publishers are on this bandwagon now. With the advent of watermarked PDFs and reasonable cost low-volume publishing, you can get a world of games from anywhere, often for the cost of a movie ticket. Most of the publisher's in @mxyzplk answer are now on the PDF bandwagon and they've opened up much of their back catalogs as well.

As a lazy DM in 2016, the amount of Adventure material available is vast and ever increasing. I recently glued together a quest from a couple of maps I downloaded for free and a fun book of NPCs. I extended the adventure with a paid-for "Dungeons on Demand". That has extended into another one from the same series and the universe is kind of building itself around a few other adventures I have purchased. At a few dollars a piece, it's no longer the end of world if you buy something that's only partially useful.

The Pathfinder RPG from Paizo (a system based on D&D 3.5, but with updated rules, and a setting information, so depending on your players reasons for "no d&d" may be no good, or may be fine) has many, many published adventures, typically organised into adventure paths, and intended to be run one after the other to form a complete campaign arc.

For the most part game companies have stopped publishing adventures as only one person in each gaming group typically buys them. Wizards of the Coast mentioned this about the time 4e came out when people asked about that. Steve Jackson games states on one of their web pages that electronic publishing is the only way to put out adventures profitably.

Player materials on the other hand are typically bought by everyone at the table, including the DM. Given that the average table has about 6 people you can see why adventures are no longer very popular among publishers.

Dungeonslayers is a light, modern RPG with a strong line up of dungeons. However they are masmorras. Only a few pages, if not one page, all combat and traps, all the time.

As mentioned Steve Jackson games apparently has online adventures out, but I have no idea how 'pick up and go' they are.

To answer the question why the industry doesn't publish terribly many adventures (in ratio to other supplements/core books) is because adventures just don't sell as well. Whereas the corebook has a decent chance of being bought by every player in the game (or at least most) an adventure is only going to be bought by two subgroups: DM's (who be their nature are 1/xth of the group) or collectors. Going on the assumption that most RPGs are just marginally profitable ventures then you can see where this quickly leads to: little or no financial incentive to publish adventures. This was actually part of the reasoning behind the OGL for D&D 3.0. WOTC didn't want to have to publish many adventures so they figured they'd sort of "open-source" the system and let the little guys do that.

Recently digital formats are breathing some life back into adventures as they can be cheaply published via the web. As you can see from other answers this is a fairly popular route. I'll add into that White Wolf's Exalted and Scion both have digital adventures (technically they are called SAS - Storyteller Adventure System I think) so if you are familiar with White Wolf but don't want World of Darkness I'd recommend checking them out.

I believe now Eclipse Phase might have a con adventure or two published as well.

Edit: Just occurred to me that I neglected to mention Goodman Games and their Dungeon Crawl Classics line for D&D 3.x and 4.0. They put out over 50 of them for 3.x alone (though I don't know how many for 4.0).

Dungeons & Dragons 4th Edition has a huge backlog of adventures available via their Masmorra revista digital. However, you must subscribe to Dungeons & Dragons Insider to access this content.

Also, if your group doesn't want to play D&D because you thought 3rd Edition was too complex, you realize that means you are the target audience for 4E, right? :)

The best use of adventures these days is as advertising. To that end, you can find quite a few with the quick play rules of various game systems. This is why they are also given out On Free RPG day. For an example see: Lenda dos Cinco Anéis.

The other avenue is digital distribution, and the only digital adventures I'm familiar with are in D&D 4e's Dungeon Magazine, sorry.

Adventures as we know them are only good as books. They need a good story so the GM is enthralled, but then they provide a fixed plot. If the players go outside of what's written on the book, they're useless. The direct consequence is railroading*, which is wresting decisions out of the players' hands.

This won't stop people from selling adventures, but is giving birth to a lot of new games that can't be run with an adventure (most will need scenarioes instead, which are easier to create and are not worth a separate book).

* Or maybe participationism, where everyone agrees to stay on the track.