Teeworlds Online 2D Shooter

Teeworlds is an open source 2D platform shooter featuring online play. Your round character will run, jump, and grapple around maps while wielding weapons and avoiding being killed by other players.

The game recently was forced to change their name because of a copyright issue.

Officially, there are no packages available for Ubuntu. However, there is a repository available (forum thread here). Open System->Administation->Software Sources. Under the Third-Party Software tab, add this source: