Once intended to be a commercially sold game, the 3D space shooter No Gravity (previously known as Space Girl) from realtech VR is now available under the GNU GPL. While there is no GNU/Linux version of the game, the source is available right now and a MacOS X port has already been done, which hopefully means that ports to other UNIX-style systems might be fairly easy to complete. According to the licensing, the data file necessary to play is not currently redistributable except via the official Sourceforge site. However, the developer says this may change in the future, with realtech VR retaining the copyright but allowing unmodified archives of the data files to be distributed by anyone in the future.
Source: Liberated Games