BioShock 2 first came out on consoles and PC -- but, oddly enough, the game's still not finished being released. Next month, the game finally gets ported to Mac OS X and, like the first BioShock, the work is being done by Mac dev Feral Interactive, who will also handle distribution of the game online.
Maybe you're staunchly opposed to buying games from Feral Interactive's website, for fear of the game being untrainable and potentially disease-ridden? Gamersgate will also be offering the game as a bundle with the first, so you can totally chill out. BioShock 2 for OS X will become available at some point next month, but if you really want to give Feral your money early the company is already accepting pre-orders.