Cube

Cube is a landscape style engine that pretends to be a FPS. This game is probably one of the best open source FPSs currently around. There is no plot, but it makes up for it with lots of monsters, a good AI (or it can be a dumb one if you want), and multi-player support. One of the most interesting features of cube is cooperative editing, so you and your friends can create levels together.

The overall feel is a bit like Quake 3 Arena. The monsters vary deeply, some look like enhanced monsters from Doom, some look like giant knights that fire very slow moving blue balls, others look like the devil, some look like those little annoying monsters from Halo.

Cube has since been dated, and a redesign of it has produced a new project, Sauerbraten or (Cube 2). You can install both on the same system, which is good, since they acknowledge that the original Cube is smaller, faster, and probably easier to understand for all those looking through its source code. Other then that, they are both primarily game engines lacking any real storyline (like anyone actually needs those..)