SecureSkat is a peer-to-peer implementation of the German card game Skat. The program negotiates participating players and game sessions over an arbitrary IRC network (Internet Relay Chat). Neither a trusted third-party (dealer) nor a centralized game server is necessary. All critical operations, e.g. shuffle of the deck, are performed using advanced cryptographic techniques (so-called Mental Poker solutions) provided by LibTMCG. The outdated OpenSkat branch contains a graphical user interface based on XSkat 3.4. This interface can be used with most recent versions, but must be built separately.
gGo is a Go board, SGF editor, client for the Internet Go Server, and an interface for playing with GNU Go. gGo is written in Java and available for Linux, OS X, OS/2 and Windows. gGo is distributed free of charge. Go is an ancient boardgame, very common in Japan, China, and Korea.
Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.
You are captain of the cargo ship Chromium B.S.U., responsible for delivering supplies to our troops on the front line. Your ship has a small fleet of robotic fighters which you control from the relative safety of the Chromium vessel. Chromium B.S.U. is a fast paced, arcade-style, top-scrolling shooter. It uses OpenGL for graphics, OpenAL/SDL_mixer for sound effects, GLC/FTGL for text rendering, and glpng/SDL_image for texture loading.
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.
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.