I used rattleCAD to design my last steel tubed frame. It was easy to use and free. Highly recommended. It won't help you define carbon lay ups and what have you but it does a great job with geometry and for round tube builders, it's pretty much all you need. When mitering and cutting tubes I lay the tubes on top of the drawing and compare against the full size drawing I had printed. One caution: after printing measure the drawing to make sure the scaling is correct. I had to print a couple of times to correct scaling problems.
Prior to using rattleCAD I made a full size drawing by drawing it myself. I didn't do the rear triangle other than figure the axle point, just the front triangle. It's easy and fun to draw. Again, lay the frame on the drawing during construction to make sure you don't stray off the design too much. Good fun.