Steam Content Delivery
is a game delivery system that allows users to purchase games in an entirely digital form. Games are associated with your account and can be downloaded to any system with a (speedy) Internet connection with patching and updates taken care of automatically.
A Mac OS X client was released on 12 May 2010 allowing any "PC" purchased games (if available) to be playable on the Mac. One annoyance is that the Steam application installs your content to ~/Documents/Steam Content. If you're anything like me I only want proper work files in there (as it's backed up to Amazon S3) so multi-gigabyte increases are not welcome. As the application has no way to change this you'll need to create a symlink. I moved mine to my downloads folder as follows:
mv ~/Documents/Steam \Content /Downloads
ln -s /Downloads/Steam\ Content ~/Documents/Steam\ Content