Teeworlds

The Teeworlds site describes it as a "retro multiplayer shooter."

I think of it like a 2D, cartoon Quake-lite. You play Tees, cute little balls who hop about destroying each other in various fashions.

Your weapons include the rifle, shotgun, rocket launcher, hammer, and laser. Most maps also include a special, limited-time upgrade that turns your Tee into a ninja.