Games
StepMania
StepMania is an extremely popular, open-source, MIT licensed game, very similar to Dance Dance Revolution. StepMania enjoys an enormous user base, and is under active development, leading to a highly polished, very playable dance game.
Free, open-source, available for many platforms
Frets On Fire
Frets On Fire is an extremely popular, open-source, GPL licensed game, very similar to Guitar Hero. This game also enjoys a large user base and is under active development, and is also polished and very playable. The Frets On Fire community has produced hundreds of songs for use in the game, and the game comes with an integrated tool for importing Guitar Hero songs; players may compete against each other on an online score chart.
Free, open-source, available for many platforms
GLtron
GLTron is an open-source, GPL licensed lightcycle game, very much like that featured in the movie, Tron. Players may compete against computer-controlled players, or real people on the same keyboard.
Free, open-source, available for many platforms
Armagatron
Armagatron is also an open-source, GPL licensed lightcycle game, very much like that featured in the movie, Tron. However, with Armagatron, players may compete against computer-controlled players, or real people over the internet.
Free, open-source, available for many platforms
LBreakout2
LBreakout2 is an open-source, GPL licensed breakout-style game, with hundreds of custom level sets created by members of its large user base. It is under active development, and is polished and very playable.
Free, open-source, available for many platforms
3D Astronomy and Planetarium Software
Stellarium
Stellarium is an open-source, GPL licensed, full-featured planetarium for your PC. Stellarium is very sophisticated, polished, and enjoys a large user base and active development, so the community contributions are broad and numerous. The developers of Stellarium place emphasis on realism, which is one reason Stellarium is in use by the projectors of real, brick-and-mortar planetariums around the world.
Free, open-source, available for many platforms
Celestia
Celestia is also an open-source, GPL licensed, full-featured planetarium for your PC. Highly polished, very sophisticated, and extremely accurate, Celestia enjoys a large user base, including some high-profile users such as NASA, ESA, and CBS's NCIS. Celestia has a smaller catalog of stars and objects than Stellarium, but it also enjoys a large user base and active development, so the community contributions are broad and numerous.
Free, open-source, available for many platforms
