a couple of open source games i recently found or have been signaled to me on IRC:
NAEV: a remake of Escape Velocity
It lacks a bit on contents, but the game engine is solid and the trading is fun.
http://code.google.com/p/naev/wiki/Index?tm=6
Rigs of Rods: A racing car, truck, boat and flying simulator with realistic physics. Name a vehicle type, it's in there.
The game is also highly moddable with more than 2000 active mods.
http://wiki.rigsofrods.com/pages/Portal
Vega Strike: a 3D space simulator game, basically a remake of Privateer.
http://vegastrike.sourceforge.net/about.php