www.fun-motion.com
They arent all freeware, but those that are are quite fun
Other than that, theres Cortex Command (a sadly defunct sidescrolling shooter which would have been awsome if it were ever finished), Shipgame (the same, I dont think you can download it now though), and the Falling Sand Game (more of a toy, but still fun).
I know The Underdogs (might get in trouble for linking to it...) has some good stuff in terms of abandonware, and theyre generally careful in terms of legality, as the ESA watches their site closely.
I know of several cheap games ($20-10) out there as well. Children of the Nile, any of the Hitman games, and X3 spring to mind. Oh, and The Elder Scrolls 3: Morrowind is a must buy.
Also, if you can find them, games like Die by the Sword, Total Annihilation and Freespace 2 are good buys. They arent uber in terms of graphics, but their gameplay will blow your mind. TA particularly, it was published in 96 I think, and still has an active fan community making tons of mods for it. Sadly, that community is in decline now.