The old stuff is fun, but I've gotta recommend the newer efforts for really high quality text adventure. The current interactive fiction subculture is doing some pretty keen stuff, both novel and retro-flavored.
Easily one of the best detective games ever created, and also my favorite Infocom game. Time passes as the game goes by, things happen in the game world independent of the player, multiple-endings, its got it all.
HellMOO (http://www.hellmoo.org) is a fun MUD I've played on and off over the years. If you're in to post-apocalyptic scenarios and pop culture references, this game is for you.
I think Emily Short's list of recommended IF (http://emshort.wordpress.com/how-to-play/reading-if/) is very good since it breaks up games by category. Nick Monfort also has some good recommendations (http://nickm.com/if/rec.html).
Personally, if you're looking for the old time feel, I would recommend:
Anchorhead (big, Cthulhoid, genuinely chilling in places): http://ifdb.tads.org/viewgame?id=op0uw1gn1tjqmjt7 Christminster (puzzle-oriented, very tough): http://ifdb.tads.org/viewgame?id=fq26p07f48ckfror Curses (puzzle-oriented, great setting, time travel): http://ifdb.tads.org/viewgame?id=plvzam05bmz3enh8
How to play these: http://www.ifwiki.org/index.php/FAQ#How_can_I_download_and_p... Easy entry point: http://pr-if.org/play/