In the past I used TurboCAD for loudspeaker designs. Unfortunately my TurboCAD version is extremely old and also does not run on my Mac. Therefore, I was looking for a new CAD software. Sketchup is good for some designs, but has two major problems: It is not designed for the creation of small parts (it is not usable for sub-millimeter resolution) and it does not support boolean operations in 3D (the commercial version does, but not the free version).
After looking at several open source CAD systems I decided to start using FreeCAD. FreeCAD is extremely powerful, it supports even parametric modeling. Unfortunately this also means that you need some time to learn how to use it. FreeCAD is available for Windows, Linux and Mac. Unfortunately, the Mac version is a bit less stable than the Linux and Windows version.
There is another problem for Mac users: You need a third mouse key. MagicPrefs worked well with my Magic Mouse to implement this on MacOS.
Update 16.06.2013: The parametric modeling is extremely powerful if you understand how it works. It makes changes on existing components very easy. Unfortunately the MacOS version of FreeCAD has some stability issues. Therefore save often or use the Linux version in a virtual machine.