As primarily a Mac user I thought I should start a thread to help Mac users get started now we can use OS X end to end for our toolpath using arduino/USB based controllers. Please help by adding to the list. Also, should we make this topic sticky or a wiki?
CAD (always free):
Blender (not great for real CAD but decent for curves moelling http://www.blender.org/
PyGear (Good for gears) http://sourceforge.net/projects/pygear/
Autodesk 123D (seems like it will stay free) http://www.123dapp.com/design
CAD (currently free)
Get 'em while their beta - these apps are pretty stable and are free at least while they are still works in progress)
Rhino3D - the best NURBS modeller there is http://www.rhino3d.com/
Inventor Fusion (Check the App store - you need an Autodesk account to activate)
For CAM the options are thin on the ground for Mac but look as PyCAM http://sourceforge.net/projects/pycam/
and also PartKam https://launchpadlibrarian.net/58759580/partkam005.swf which is actually a flash-based CAM app. I've not used it but looked at it and flash makes it almost platform independent - you can run it on your Android Tab/Phone even.
Since you are probably using GRBL at the moment see the wiki for some cross platform GUIs and options (python and java options currently available)