Maybe you've heard of those "Humble Indie Bundles" they've been periodically releasing the last few years. Apparently the latest one just dropped, featuring a pack of games by indie British game developer Introversion. The basic model is that you pay however much you want for the games (including nothing) and select how much you want to go to the developers, two charities, and the site itself. To foster a spirit of competition, it tracks the donations by OS (currently Windows is the most tight-fisted, and Linux users are the most generous) and the top donors. It seems like a pretty cool business model and I recommend checking it out.
As for the games themselves, I've tried a few and they've been worth my payment. Crayon Physics is a pretty brilliant sandbox-puzzle game that lets you literally draw in your solution and let the game's realistic physics engine take over. Reminds me of a video I saw a few years ago of similar drawing-based physics being demoed on a whiteboard, which I thought was the coolest thing ever at the time. It's especially great when played with my Wacom tablet. To get it and one other game, you need to donate at least the average donation, another smart touch. I've also tried Darwinia, an odd RTS-type game with similar aesthetics to Tron or Synaesthete, and a Windows-only demo of a procedural city generator that creates lifelike street plots of cities that will never exist in seconds. Very shallow and not a full game, but promising. I have yet to try DEFCON, a satirical strategy game simulating the cold war, and Uplink, which apparently simulates Hollywood-style hacking. If you're into smart, offbeat-type games, I would follow this site. (It's free if you want, after all)