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.
The Mana World (TMW) is a serious effort to create an innovative free and open source MMORPG. It uses 2D graphics and aims to create a large and diverse interactive world. The game aims to be a unique place for people to interact not only by fighting each other but to play life, form a community within the community, or just simply chat. Although the game is never-ending and a linear story cannot be applied, it should have enough background to describe and predefine tribes, kingdoms, and the environment overall. Fullfilled quests should lead to certain events depending on them being solved. Quests should also encourage teamwork.