Let the gaming begin! Valve Software on Wednesday publicly released Steam for the Mac. It’s now available, and it’s free to use, though most games incur a fee to play. Valve is also giving away its popular action game Portal until May 24, 2010.

Steam provides a framework for buying and downloading games online. It’s been around since 2004 for Windows PCs, and Valve has finally released it for the Mac. The service requires Intel-based Macs; games have varying system requirements.

While the vast majority of games available for Steam work on the PC only, Steam is launching on the Mac with more than 60 titles. The majority of the launch titles are casual games – puzzle games, task management games and other types of games that can already be found through services like Macgamestore.com, though there are a few surprises for hardcore game fans, such as Portal, Civilization IV and several expansion packs, and Torchlight, a dungeon crawl action game reminiscent of Diablo.

The Mac release of Steam coincides with the introduction of a new feature on Steam called Steam Play. Steam Play lets users buy a game once, but play it on whatever platform they prefer, Windows or Mac. Not all games are available for both platforms, but those that are only require users to buy one license to play on either platform (or both, if they’re playing a game at work on a PC, and home on a Mac, for example).

Steam Play also extends the concept of Steam Cloud, an existing feature to Steam that enables players to seamlessly pick up where they left off in a game, regardless of location. Changes to game options and save files are mirrored wherever you log on to Steam.