I'm using AutoCad from work. It's pretty crude 2D, but I find I can "sketch" much faster than I can in 3D. 3D obviously has advantages for detailed design, but just to lay things out, get a general idea of how things work, I like AutoCad.
Just a note, a few years ago I bought an official version of ProE 2000i Student Edition for like $40 on Ebay. That's another way to go. The Student Edition means that you can't ever export one of your drawings to the full version, you're always locked into a student version "world". But other than that, I don't think it's any different. I've used it to design some parts, print them out, it prints "student edition" on the bottom. NBD.