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 Tunnels and 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.