If you like horror, you could try the Chzo Mythos games. The Special Editions, with stuff like author commentary and 'deleted' scenes, is now freeware.
The first game in the series is here:
http://www.fullyramblomatic.com/5days/
It's not freeware, but a pretty good obscure adventure game is Inherit the Earth: Quest for the Orb. You're in a medieval (or IS it?) world populated by anthropomorphic animals, and you're on a quest to find a stolen orb that can control the weather. It plays a lot like a Lucasarts game, if you're into those.
Buy here:
http://wyrmkeep.com/ If you beat the game, read the webcomic, it's a sequel.