PlAr (PLatform ARena) is a free and open-source multiplayer scrolling game where uou mobe the characters through a scrolling level, plus you can fight with other humans. The goal is to play in the 2D style against other humans.
You will be able to select to play with three different characters, they can all run and jump and fire, in the 2d platform*s, plus each of one have a personalized special skills that you activate with the ALT or ALT GR key.
Detailed instructions on how to play PlAr on your Mac are available HERE.
PlAr is a cross-platform utility capable of running on any operating system that comes with Java support (e.g. Mac OS X, Windows, Linux).