1Up.com is running a story called The Essential 50, which lists what they consider to be the 50 most essential video games. It's an interesting history lesson, since every entry has a short history of the game, the innovations it had, and the impact it had on the industry and video game development. Interestingly, several games' major impact was not technology or innovation, as I would have imagined, but rather political and/or social. Interesting stuff.