Bricksmith is a cool app that let's you tinker with a set of virtual Legos on your Mac. How cool is that? Plus, it is now a Universal Binary so Intel and PowerPC Macs can join in the fun, natively. Check out the tutorial to see all the cool stuff you can do with Bricksmith.
Bricksmith is donationware, so if you find yourself heavily using it kick in a few bucks to support the project.