A good open source platform game that never gets a mention is The Goonies: 20th Anniversary Edition. This game is a remake of the classic NES / Master System version by Konami. The same team is also responsible two rather fun arcade top-down racing games, F1 Spirit and Road Fighter, which are also remakes. I'm not sure what the source license is for these (ack, there goes my Free Software credibility) but you can get the source and the games compile / run on the 3 major platforms (y'know, Lin/Win/Mac).