Valve has announced that the first games available for Steam on Mac tomorrow will be Portal and Torchlight.
The former should need no introduction, but the latter is a Diablo-alike RPG from Runic Games. It's good.
New games will follow every Wednesday from now on - "each designed to highlight specific functionalities of Steam on the Mac".
Mac Steam uses Steam Play. This allows users of PC Steam to play their existing purchases on Mac - handy for people with both platforms or migrating from one to the other.
Valve also announced that native OS/X support for the Source engine is available now, as is the Steamworks suite of tools.