Open Sonic is a free and open-source 2D platform game that is based on the classic Sonic the Hedgehog game series by Sega. The game features Sonic-style physics with a 360° movement engine, availability in 9 languages, ability to create your own levels using the built-in level editor, and much more.
Mac OS X 10.4 or newer
Intel or PowerPC powered Mac
Free download at opensnc.sourceforge.net