Adventure Game Studio [http://www.bigbluecup.com/] is a great resource for freeware retro adventure games. Sure, there's a lot of crap in that database, but there are also some absolutely fantastic games available. Some of my favorites:
Charlie Foxtrot and the Galaxy of Tomorrow [http://www.bigbluecup.com/games.php?action=detail&id=906] - a pretty funny sci-fi parody game that owes a lot to the old Space Quest games. This one is a particularly good choice if you're new to adventure games.
Ben There, Dan That [http://www.bigbluecup.com/games.php?action=detail&id=1056] - another good game made great by some truly fantastic writing; much funnier than most professional "comedic" video games.
Reactor 09 [http://www.bigbluecup.com/games.php?action=detail&id=819] - a beautifully rendered sci-fi quest with a pretty engaging plot. I can't really put my finger on why I like this one so much, but I do, so give it a shot.
The Vacuum [http://www.bigbluecup.com/games.php?category=&action=detail&id=1066&refresh1223961216] - I made this game myself, so you should probably play it. Some people liked it, some didn't, but those that liked it seemed to like it a lot.
There's also the Chzo saga games that Yahtzee made before he started doing Zero Punctuation, but it's probably easier to get them from fullyramblomatic.com. Trilby's Notes in particular is fantastic, but make sure you play them in the proper order.
Ok, maybe that is a bit much, but still it is unbelievably good. If you try it, just remember not to use any kind of extra graphics. ASCII is how it is meant to be played.
