Tgatb is a 2/3D client for a MMORPG using SDL libraries, written in C, so it should compile at least under Linux, Windows and Mac OS. The game server is up and running, so you can log into the game and start playing. A client for Linux and Windows is available.
XQF is a game server browser and launcher for Unix/X11 written using the GTK toolkit. XQF is a front-end to QStat and supports many games including Quake, Quake2, Quake3 and Unreal Tournament.
XQF development is now tracked on GitHub: https://github.com/XQF/xqf
The NNGS (No Name Go Server) is a game server which allows you to play the game of Go over the Internet. It basically runs as a 'deamon' under Unix (though cygwin-port is available) and supports connections from many clients (both graphical and text)