GamersGate just released VOID. It allows you to download and play PC games for free. The game selection is pretty small right now, but there are some good ones like Risen, Men of War, and Majesty. After installing the game of your choice, when you run it, an ad or two (about 60 seconds total) will play and then the game will start. Simple as that!
Read about slots on the VOID page to learn more about how it works. Your first slot, not upgraded, will take two weeks before you can put another game in it.