Cube Sauerbraten 2
Free single and multi player 1st person shooter game with some satisfying fast oldskool gameplay.
A large variety of gameplay modes from classic SP to fast 1 on 1 MP and objective based teamplay, with a great variety of original maps to play on.
Level editing has never been so much fun: a press of a key allows you to modify the geometry / textures / entities in-game, on the fly. Even more novel, you can make maps together with others online, in the unique "coop edit" mode (!)
The engine, though designed for simplicity and elegance as opposed to feature & eyecandy checklists, still competes nicely thanks to its novel "6-directional heighfield deformable cube octree" world structure that is the basis for its in-game editing. Occlusion culling, pixel & vertex shaders, very accurate lightmapping, robust custom physics system, network system, models, sound, scripting...
Mac OS X 10.3.9 or higher.