Let's see, we have Wasteland. Dated graphics, but very very solid gameplay. Spend an hour with it, and the graphics will no longer bother you, and your life will be enriched.
If you liked wasteland, then you are in luck, because it got a sequel: Fountain of Dreams. This game is not as groundbreakingly amazing, but still a solid game. (CRPG)
If you want the postapocalyptic feel, but in a fantasy setting, then check out Dark sun. Not the best games ever, but solid (and darksun has bands of ravening flesh eating gnomes, yea take that cookie cutter common AD&D gnomes!) (CRPG)
Armageddon empires has already been mentioned. Check it out, it is great! (It is a strategy game)
We also have UFO: Aftershock & UFO: Aftermath, again not amazing, but still solid. You can get the entire UFO trilogy for next to nothing these days. (These are tactics games)
Abomination is worth checking out as well. Not as good as X-com, but still solid (Again, tactics games)
Chamber of the sci-mutant priest has a bit of a fan following. Not played, but apparently it is supposed to be rather good (CRPG)
Dark Earth is a really solid action game, that again has quite a fan following. (Action-adventure)
Warzone 2100 is one of the finest RTSs around, and it is absolutely free [http://wz2100.net/download]. (RTS)
And that should be enough for now.