Not sure how I missed this, but ChronicLogic has released The Bridge Construction Set (aka Pontifex II) for OS X! I played various versions of this game at Game Developer Conference in 2003 and 2004, but since I don't own a Windows machine, I've never been able to have my copy. Now I can. Woo hoo!
Like the name implies, this is a bridge building game. It's not fast-paced action, but a physics simulation for bridge design. (I guess that makes it's an Educational game.) You are given certain amounts of materials, land that needs a bridge, and stress/load testing to watch it crumble. Building simple, strong bridges tends not to be such a difficult task -- but exploring one's creativity in designing completely absurd but functional structures is where the fun is!