I'd recommend Draft Sight too, it's completely free but only 2D though.
CLICKY
If you need 3D there are a lot of options, these are all free !
Wings 3D
http://www.wings3d.com/
True Space (possibly PC only, can't you run it through Bootcamp though ?)
http://www.caligari.com/
E Drawings -
http://www.edrawingsviewer.com/
Free CAD -
http://www.ar-cad.com/freecad/index.html
Or there are low cost programs like Cheetah 3D
http://www.mac3dsoftware.com/
And many more which I've lost my list for so you'll have to google them yourself.
I think the main thing is to work out what you want to do with it, if it's to output files you need to speak with the people who you're outputting them too because when I was looking for this sort of thing at my last job a lot of the cheap or free ones wouldn't output in the right file format. So in the end Modo was chosen but it's £800...