Cuprum 4+

Cuprum is a multi-layer Gerber, Excellon and ODB++ viewer for the Mac platform.

It has basic editing capabilities and can export to Gerber X2 and Excellon as well.
Multiple files of different kinds can be combined and exported as a single Gerber dataset.

It supports viewing (and printing) multiple layers at the same time.

A QuickLook plugin is integrated in the app, so you can preview files in the Finder and other apps.

Features (some require an In App Purchase):
-Reads Gerber, ODB++, Excellon and XNC files
-Supports Gerber X2
-Compare images to quickly spot differences between two dataset revisions
-Various tools tool with advanced snap feature.
-Cuprum allows full zooming and navigation of the image with an intuitive interface whether you use a trackpad, magic mouse, normal mouse or pen tablet. Even your 3Dconnnexion 3D mouse is supported!
-It can also zoom to actual size, which corrects for different monitor DPI’s, so actual size is actually actual size.
-Multi-page print out (or PDF generation via the print panel) is supported with variable scaling and presets for actual size and page fill. Frames can be added, which also supports string substitution.
-A QuickLook plugin is integrated which allows Finder to show a preview of a document.
-Selection of objects, allowing inspection (and editing) of all properties.
-Highlighting of connected copper.