I agree this has the feel of being a popularity contest or mindshare list. There are plenty of developers that have contributed to development of gaming in different ways.
Valve should definitely be mentioned for their contribution to gaming. Quite of few of you guys seem to forget Valve's other accomplishments both gaming and technical. They more or less kept PC gaming viable and one of the few that is pushing gaming to platforms other than Windows on PC. What about TF2 which as had plenty of free content and updates that other games would have charged for as DLC. What about Counter-Strike which is depending on what stats you track either the #2 or #3 most played FPS on PC after Call of Duty and Battlefield.