Orbital Rising is a sci-fi mech FPS/RPG, with customization similar to Armored Core, in addition to unique story elements. It is built with the Unity engine.
This game's source has been opened, in addition to most of its development documents.
A demo of the game can be found here, for Windows only. A Mac port can be built upon request. Be sure to highlight and select the target with RMB, and fire with LMB. http://bitbucket.org/cryptoquick/orbital/downloads/OrbitalRisingDemo1.zip