Developer: Gearbox Software
Release Date: November 19, 1999
Source Code Release Date: Not released to the public
Half-Life: Opposing Force is the first expansion pack for Half-Life. It takes place during the events of the original game, but instead of playing as Gordon Freeman having a bad day at work you play from the perspective of one of the Marines sent into the complex. It was definitely an interesting maneuver, and one that avoided having to explain what happened after the end of the first game, they wouldn't go there until the sequel.
NOTE: the support for Opposing Force as a Xash3D-FWGS mod is still in development. You may notice quirks like glitches with weapon animations. For this reason I have labeled the game as "Early Access". If you're willing to ignore the issues you can go ahead and experience the expansion pack on your Mac again.