The best games are at POPCAP.com!
Download.com has many game and for kid !
http://the-underdogs.org is a website dedicated to preserving good games that have entered into the public domain. Like System Shock 2 and the Freespace series. They also provide links to websites that sell out of print games.
i'm so boring and need some game!
Not a large collection, seeing as this is just a single organization, but I love some of these games (especially Icy Tower): http://www.freelunchdesign.com/