Eager gamers have no doubt already seen the teasers, but Valve has now finally confirmed that its Steam game distribution service and Source engine will at long last be headed to the Mac. According to Valve, the company's current line-up of games (including the Half-Life and Left 4 Dead series) will be available to Mac users in April, while Portal 2 will represent the company's first simultaneous release for PC and Mac later this year. Better still, Steamworks for Mac also boasts a new feature called "Steam Play," which will let you buy a game once and, for instance, start playing it on a PC at work and then pick up where you left off on your Mac at home (we'll give you a moment to let that sink in). Game publishers will have to enable that feature themselves, but Valve says it expects most to take advantage of it.