Valve has released another Steam Play Mac title, and this time it's Garry's Mod, the wild sandbox physics editor that uses assets from Half Life 2 to create whatever you want. The mod is probably most famous for its Rube Goldberg-style creations, but it's also very handy for machinima and other graphic model design as well. It's not so much a game as a way to just create all kinds of wacky machines and computer characters, and have them interact in whatever way you'd like.
If you already own it for the PC, then you own the Mac version as well, so you can head into Steam on OS X and download away. If you don't own it yet, Steam has a 50% off discount up right now, so you can pick it up for just $4.99.