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.
QuakeWorld/2 is a port of QuakeWorld & Quake to OS/2 2.0 The work is done with EMX, and the goal is to run under OS/2 2.0 along with the IBM TCP/IP that runs on OS/2 2.0 .