While there are a few viewing tools available for OSX, I’ve been looking for something that was higher quality, and more configurable. Today I stumbled upon Cuprum. This viewer supports layering, export of user-configured views, some good print options and other stuff. It’s $13 on the Mac App store, and well worth it IMHO.
Here’s an example of one of my breakout board shields. Overall this makes review of a gerber output MUCH easier than other programs I’ve tried. An app store DL is available here – It’s $14,99 on the app store.