From April onwards Mac-users will be able to use Valve's Steam platform natively on OS X. Using Steam on OS X was possible before by using Wine but compatibility was varying. Wine is a set of open source Windows Libraries ported over to Linux/OS X allowing Windows applications and games to run natively within the Linux/OS X environment. Together with the appearance of a Native OS X Steam variant the recently announced Portal-2 will appear on Steam simultaneously in both PC- and OS X flavors.

Rumors about Steam coming to OS X were going around for some time and when a new beta release of Steam contained references to OS X the rumor mill churned full speed. Last week screenshots appeared on-line showing the OS X specific features of the interface. And finally last Monday Valve made the official announcement. Apple users will get access to newly compiled / ported versions of the games and won't be able to buy/run Windows-only games. PC users can rest assured that they will continue to have access to a bigger library to choose from. Games Mac users will also be able to choose from are the Half-life series, Counter-Strike, Portal and Left4Dead. Not sure if the entire library will eventually be 'maccanized'.

Developers using the Steamworks-API are actually able to make games available on both OSes, so people migrating from one type of system to another can still play their games. At least of the companies are willing to make this available, technically it is possible and Valve believes publishers will actually choose to do so. OS X and Windows games will be compatible in multi-player environments and save games will be transferable between both OS X and PC versions. Games supporting Steam-Cloud will actually synchronize automatically.

The porting of Steam to OS X was largely possible due to the fact that Valve opted to ditch Internet Explorer Technology and adopted Webkit as the engine behind Steams user-interface. The games ported to OS X are using OpenGL as the graphics-mode whereas most Windows counterparts are running DirectX code.

Of course CrossoverGames (based on Wine) allows a Mac-user to run Windows-only games too and could be an alternative. But that does require a bit more IT-knowledge than the average user. I myself have been running quite a few Steam-distributed Windows games on OS X this way. Running DirectX compatible games is very doable - at least when they are compatible.