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Strafe is a retro styled pixel art shooter in the Unity engine that purports to offer endless play with procedurally generated levels.

A re-creation of the original Half-Life in the 2007 version of the Source engine, with upgraded high definition audio and video-- up until the Xen chapter, at least. There's no native OSX version, but if you install Steam into a WINE wrapper, you can use it to install the 2007 Source SDK and play it on OSX.

The precursor to the Bioshock franchise and the sequel to the classic original System Shock, Irrational's best game ever features the only crazy AI who could give Marathon's Durandal a run for his money: Shodan. 
 
UPDATE: GOG has System Shock 2 on sale right now for $2.

This prequel takes place 1,000 years before the first Myth game and tells the story of the hero of the Wolf Age, Connacht. Unlike Myth 1 and 2 which used sprites for units, Myth 3 uses polygonal models.