Another abandonware site with similar games to the first one posted - classic games that didn't sell well enough to keep the company afloat, but were still brilliant -
Home of The Underdogs. You can get some fantastic games here, I'm now replaying Dungeon Keeper after about seven years of waiting.
The Independant Games Festival has links to a bunch of games without publishers - these are usually free.
One game in particular from this year's lineup that has got me addicted -
N. You play a small ninja in a game that's similar to Lode Runner, only instead of making holes in the floor you can jump around and off walls. And you need to, because the world is populated by a lot of killer robots. 300 levels, lots of deaths (my death count is closing in on 35,000), lots of fun. Well worth trying out (And it's completely free).