PixelBoy: Simply downloading AGS library goes over 4000.
MysterD: Question, since I'm probably out of the loop here.
What's the AGS Library?
What's AGS stand for?
As Themken already answered, it's
Adventure Game Studio, a (relatively) easy-to-use engine for traditional adventure games.
If you download everything on AGS site, you will get thousands of games. They used to be available as a torrent too, not sure if it's still available.
You can also find many games on Archive.org
Adventure Game Studio Depository, but it's far from complete, it has only a little over 2000 games at the moment.
http://archive.org/details/adventuregamestudio There are many commercial games using AGS too, like most
Wadjet Eye games and such.
Other commercial games you can also get from GOG are
Detective Gallo,
Zniw Adventure, and about a week ago added
Sumatra: Fate of Yandi, to name a few.
The best freeware stuff is quite as good as those commercial games, and there's plenty of games which are very good but might be lacking some polish that those commercial releases have.
Some people have used that engine to create non-adventure games too. For instance, Red Hot Overdrive that I played a while ago, is quite a good classic OutRun clone.
http://www.adventuregamestudio.co.uk/site/games/game/1832-red-hot-overdrive/
The best part is that you can run many AGS games with ScummVM, although oldest games are not supported, and newest ones may run too slowly, but there are hundreds of games that run flawlessly.