Garry's Mod is available now for Mac. To celebrate, the app will be $5 for the next week -- that's 50 percent off the regular price. Though, remember, if you own the PC version, you automagically own the Mac port. The one caveat with Garry's Mod is that you need to own another Source Engine game before use.
In the right hands, the mod can be used to make engaging Machinima like "The Jacknife Chronicles." In our hands, it would be used to make the umpteenth version of Gman doing "Banana Phone."