BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
pydance is a dancing game written in Python, formerly named pyDDR. The idea is simple. There's a floor mat with arrows, and the game scrolls arrows up the screen to the beat while playing a song. When the arrows reach the top of the screen (not sooner and not later), the player hits the corresponding arrow on the pad, and given that it's hit on time with the beat, points are scored. Based on how well the dance is put together, s/he is graded at the end of the song. Both keyboard and mat play are supported.
The Noble Ape Simulation is a collection of a number of autonomous simulation components including a landscape simulation, biological simulation, weather simulation, sentient creature (Noble Ape) simulation, and a simple intelligent-agent scripting language (ApeScript). Noble Ape also contains a social simulation where the Noble Apes can be tracked in terms of social groups and also over many generations to explain social phenomenon to users looking to study this kind of interaction. It has been in development for more than a fifteen years.
Thousand Parsec is a turn-based space empire building game, as well as a framework for creating a similar group of games, which are often called 4X games (eXplore, eXpand, eXploit, and eXterminate). Some examples of games from which Thousand Parsec draws ideas are Reach for the Stars, Stars!, VGA Planets, Master of Orion, and Galactic Civilizations. Unlike commercial alternatives, it is designed for long games supporting universes as large as your computer can handle. It allows a high degree of player customization, and features a flexible technology system where new technologies may be introduced mid-game.