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.
Globulation is an innovative high quality real-time strategy which minimizes micro-management by automatically assigning tasks to the units. The player has to choose the number of units they want for the different tasks, and the units will do their best to satisfy the requests. This allows the player to manage more units and to focus on the strategy.
Stratagus is a cross-platform real-time strategy gaming engine. It includes support for playing over the Internet/LAN or playing a computer opponent. The engine is configurable and can be used to create games with a wide range of features specific to your needs. Some games based upon it include Battle of Survival, Wargus, and Battle for Mandicor.