Do you want good games for free without resorting to piracy? Just be patient and wait for free games from Humble Store, Steam, and GOG. The history of paid PC games that had been offered for free shows you can build your PC game library without spending money in a legit way.
All of the games listed below are available on Linux which means you can use a free OS to play free games. This is a work in progress and will be regularly updated as more information becomes available.