Jin is a Java client for various chess servers. It currently supports the Internet Chess Club (chessclub.com) and the Free Internet Chess Server (freechess.org) but is designed to be able to support any chess server.
Apricots is a quick, fun, and delightful 2D plane dogfighting game. It supports 1 or 2 players with possible computer AI opponents. Players shoot enemy planes and bomb targets with the aim of attaining the highest score.
A 3D-graphics enhanced game engine for Doom, Heretic, and Hexen.
The Doomsday Engine is an enhanced Doom source port, based on the source code of id Software's Doom and Raven Software's Heretic and Hexen. The purpose of the project is to create versions of Doom, Heretic and Hexen that feel the same as the original games but are implemented using modern techniques such as 3D graphics and client/server networking. A lot of emphasis is placed on good-looking graphics.
You can play Backgammon with other people around the world. The server moderates games via the telnet protocol (ASCII.plain-text) or via a graphical application for different platforms (e.g. JavaFIBS, MacFIBS, et.al.).