I've been following the threads about CAD by Bronxilogically Violated, and have been too apathetic to mention:
If you want to make 2D _drawings_, the kind of stuff that you actually give to machinists to make parts*, then QCad is pretty good. It's a marketing vehicle for some "full version" 2D cad system that you have to pay $$ for, but QCad itself is open source and pretty darn good**.
I use it for drawing model airplane plans, mostly, and it works just fine.
- Well, all but the most complicated parts, in which case everyone gives up on the whole 2D drawing thing and the shop just asks for your 3D model.
** Except splines suck -- one of their sales points for the commercial product, when translated from Sales to Pithy, reads "and splines don't suck".