Support for OSX and Linux was promised quite some time ago, but we are finally seeing the result of it. You could already try it as QuakeCon.
The rest of the world can enjoy it next tuesday (the 18th of august).

Sources:
http://www.techradar.com/news/gaming/quake-li...ort-626333
http://news.softpedia.com/news/Quake-Live-Get...9235.shtml