It's only been a short while after the admittedly long-in-the-making official release of Dota 2 for Windows and Valve has already prepared Mac and Linux versions of the immensely popular game. Those are available for download right now on Steam.
Mac minimum system requirements:
Linux minimum system requirements:
New players are being let into Dota 2 gradually to avoid totally overloading the game's infrastructure, so if you're among the bright-eyed beginners who are in for a world of hurt, you'll first be added to Valve's launch queue before getting into the game proper.
Dota 2 Update - July 18th, 2013 [Steam]