Valve has officially released their Steam client for Mac this afternoon. It is available for free and can be downloaded here. Some bonus content was also released to entice Mac users to jump on the Steam bandwagon.
Valve’s popular game Portal will be free on Steam now until May 24 for Windows and Mac. Hurry up and get your copy! Mac users will also notice that each Wednesday, Valve will release a new set of games playable on the Mac. The first is the feature Steam Play. Steam Play allows users to purchase a game once, then be able to play it on any supported platform. That means you can purchase one game and play it on your Mac, Windows PC, or whatever else you can use for Steam. Sweet!
Update: it seems users and having trouble purchasing and playing games. Portal seems to be giving some trouble, but it looks like Vavle is working quickly to resolve the problem!

