League of Legends has gained a large audience and tournament scene, purely on the virtues of the PC. Macs have been left out in the cold so far, but all that is changing with a newly available beta client currently in testing.
A post on the League of Legends forums (via GameSpot) announced that the the Public Beta Environment is currently running the Mac client. Riot plans to make it into an open beta once the PBE testing is finished. You can download the PBE client and check out the instructions to give feedback if you want to be in on the process.
The post says that Riot engineers re-wrote a lot of the engine to make sure that once the Mac client is live, both PC and Mac platforms will be able to get new features, bug fixes, and updates at the same time.