Netrek is a 16-player two-dimensional graphical real-time battle simulation with a Star Trek theme. A game is divided into two teams of 8 players (or fewer), who dogfight each other and attempt to conquer each other's planets.
Gibbon is a graphical frontend for FIBS, the First Internet Backgammon Server. The client is platform independent and known to compile and run under Unix, Linux, Mac OS X, and Windows.
Toga II is a chess program, derivative of Fruit created by Thomas Gaksch. It has more chess knowledge and perhaps a better search algorithm. It is based on Fruit 2.1 and is free.