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Aleph One is a 3-D first-person shooting (FPS) game engine. It's a descendent of Bungie's Marathon 2, and supports Marathon 2 and Marathon Infinity data files. A conversion of Marathon 1 called M1A1 and many third party scenarios and net maps are also available. Aleph One features software and OpenGL rendering, Internet play, Lua scripting, and more.
BZFlag is a free online cross-platform multiplayer 3-D tank battle game. The name originates from "Battle Zone capture the Flag." It runs on Windows, Mac OS X, Linux, *BSD, Irix, and many other platforms.