LucasArts also announced this morning that a bunch of their old adventure titles, including Indiana Jones and the Fate of Atlantis, as well as the excellent Loom, are also available via Steam for Mac. There's also a great pack of indie games available if that's more your speed, and Telltale also has a bundle of their Tales of Monkey Island games, along with the latest Sam and Max episode.
All of these titles are releasing under the new Steam Play program, too, which means that if you buy them on one platform, you can play them on any others as well. So if you already own Portal or Torchlight, odds are that you can simply install the Steam client (right here), and then download them to play on your Mac. Finally, after all of these years, it is an amazing day to be a Mac gamer.