Depends on what your looking for..
2D or 3D?
Are you looking to make stuff from data directly or just create drawings to print out?
The answers to those questions will give you different answers!
Free and CAD don't often cross paths, especially in 3D...and your on a mac...the worlds least supported platform for CAD...ignoring the whole mac vs pc argument...there are no mainstream/popular/proven CAD systems on mac currently...with the possible exception of TurboCAD, which has a mac version and been around for while...but not free...and used by very few if any real engineering/design businesses.
The best stuff lives on PC and UNIX based workstations and is VERY expensive!
Your best bet will be to trawl the net for shareware type stuff...but none of these will be anywhere near 'professional level' CAD.
Better bet might be bootcamp, windows and open up your options...
...but even then, the free/shareware offerings are varied and I'd guess limited...and generally not in the same league as the real thing! I'm sure people are trying to make good free stuff though so maybe I'm just a bit of CAD snob and you'll find something that works for you!
However...things to look up for those interested...
For 3D, Autodesk are currently doing a free version of Inventor Fusion...which uses a new form of 3D modelling for them. They have a "technology preview" as they call it which is free, and worth a punt....
CLICKY
Or google sketchup...which should be ok for simple stuff, and again nothing like real engineering software.
Siemens have a free 2D package by the looks of things...never used it (I've always been an AutoCAD guy for 2D), but it's free and made by a reputable company with a big 3D system, and most likely imports/exports AutoCAD formats so could also be worth a look...
CLICKY
I'd guess that a hunt around for free 2D CAD would give you a good number of options...not sure how capable they'll be, but it'd get you going at least.
These might work in parallels or VMware type environment as well, just going to be a case of experimenting I reckon!
Good luck..and let us know how you get on!