The Witcher 2: Assassins of Kings started out as a Windows PC exclusive, and now it's out on a number of platforms—including Mac OS X. The Mac version was already available on GOG and through Steam, but now The Witcher 2 has hit the Mac App Store as well, and it's extra cheap.
Not only is the game only $19.99 on the App Store—it's also the Enhanced Edition, "the most complete and uncut version," according to a press release sent out with the announcement.
That's the version that came out on Xbox 360, and it includes around four hours of extra content. So if you haven't experienced the genius of The Witcher 2 yet, it's the perfect way to do so. Just be sure to check those system requirements and make your your Mac can run it!