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.
GCompris is an educational software suite with numerous activities for children aged 2 to 10. Some of the activities are game-orientated, but nonetheless still educational. These include computer discovery (keyboard, mouse, different mouse gestures), algebra (table memory, enumeration, double entry table, mirror image), science (the canal lock, the water cycle, the submarine, electric simulation), geography (place the country on the map), games (chess, memory, connect 4, oware, sudoku), reading practice, and others (learn to tell time, puzzles of famous paintings, vector drawing, cartoon making, etc.). It currently offers in excess of 100 activities, and more are being developed.
GNU Chess lets most modern computers play a full game of chess. It has a plain terminal interface but supports visual interfaces such as X-Windows "xboard" and Windows-for-PC "winboard" as well as a full 3-dimensional wooden chess-board protocol for the Novag Chess board enabling one to be relatively free of the computer itself.
KaPooka is an animated logic game for SDL. It's kind of a cross between Sokoban and Pengo with regards to gameplay. You move your red blobby around, pushing the green blobbies out of the way and finding the exit. The game currently consists of 30 levels. Anyone that feels like contributing other levels is most welcome.
Ncohafmuta is a text-based chat server that is designed for access from a telnet client. Its purpose is to allow users to communicate in a configurable environment in realtime. It allows users to create accounts with their own settings and optional personal information. Rooms are the basis for communication, whether private or public. Other features include internal email, email forwarding, a configure script for multiple Unices, support for cygwin under Win32, macros, games, a ranking system with access to more commands, ANSI coloring, banning, async DNS, soft/hot-rebooting, detailed, organized logging, and an internal Web server port.