Cube 2: Sauerbraten is a 3D first-person shooter that features multiple single-player campaigns, online multiplayer with a variety of winning conditions, and the ability to edit maps cooperatively. Players have a choice of five different (rather whimsical) avatar appearances.
There are currently four different single-player campaigns, each quite different in style. One assigns specific objectives and narrates occurrences with printed dialogue; another provides no explicit directive, assuming the player will discover their cause in the course of exploration. All involve blasting one's way through the same beasts, in varying quantity.
Multiplayer's really where it's at, though. The game is fast-paced even with an average ping and whether it's insta-kill capture the flag or regenerative base domination, playing is an intense, fun experience that doesn't let up. Sure, there are maps where enough snipers and campers can all too easily ruin the experience for everyone, but they can be negated with a critical mass of experienced and fair players.
Sam's Protip: Zoomed aiming can be helpful, but it can also give away your position with a red line of laser sight.