If you're wanting a change from standard D&D fare, I'd recommend the following games:
Call of Cthulhu- either the BRP or D20 versions. For sheer fun and weirdness, you just can't beat CoC. It will provide you with a VERY different sort of game than D&D, and is a great way to expand your horizons.
Darwin's World 2nd edition- this meaty hardback is hands down the best rendition of a post-apocalyptic world I've ever seen. It uses D20 modern as its base, and build a compelling world that isn't quite so far-fetched at Gamma World. By the way, avoid the recent Gamma World books from S&S- they really leave a lot to be desired.
Deadlands- its campy, its western, and its horror all rolled into one! Think High Plains Drifter meets Evil Dead! This is one of the most enjoyable games I've ever played, and also very different than D&D. Every time we have played this, my group has had a blast, and can't wait to play again. The mechanics take a little getting used to, but using playing cards, dice, and poker chips really adds to the ambience and charm of this game. Avoid the D20 version of Deadlands at all costs- its horrid.
Fading Suns- a dark sci-fi game where humanity has just emerged from a dark age, and the suns are dying, religious paranoia is on the rise, and nobles, the church, and tradesmen vie with each other for power. I can't recommend the setting highly enough- its dark, evocative, and has a great deal of room for political intrugue, espionage, exploration, and horror. Its a wonderfully detailed setting crippled by terrible mechanics. The D20 version of Fading Suns is pretty good though.
The recent Mongoose version of Conan is excellent too. While there are some errors in the book, they aren't serious enough to in any way hamper playing this excellent game. Even though it is fantasy, its plays MUCH differently that D&D. If you like the Conan stories, definitely check this one out.