See here.
You sign up for the Great Games Experiment and they will send you a code or something good for one free game from the GG store. I of course signed up ages ago, so will have to make a dummy account because I also like free shit.
See the games here. Some games are not included in this deal, namely Wildlife Tycoon: Venture Africa, Fizzball, and Fastcrawl. Which totally suck because I REALLY wanted Fastcrawl. I'm going to buy that game someday...
Anyway, the GG store can be hit or miss, but I'll list my personal picks. You can also go and download the demos and try them for yourself.
JetsīnīGuns Gold: 2D side scrolling shooter. It just really cool.
Rack 'Em Up Road Trip: A kind of cool billiards game.
Minigolf Mania: minigolf, duh.
Puzzle Poker: Very cool puzzle game based on poker hands.
Tribal Trouble: RTS game with vikings vs island natives.
Gish: Squishy physics platformer
ZAP!: Geometry Wars meets Tribes. You heard me. Not sure how many still play, though.
MarbleBlast Gold: cool game, get the marble to the exit quickly