The popular DRM-free digital distribution platform Good Old Games, finally supports the Mac OS operating system - though not every game in its catalog is available at this time. GOG is currently offering 50 games on OS X including Syndicate and Wing Commander 1 & 2. More Mac-compatible titles will be released in the future on an on-going basis.
Ars Technica has a good run-down of what it is like to install a game from GOG onto a Mac. It's not necessarily a straightforward process. You can check out that lengthy report here.
Source: Ars Technica