Hello everyone, I was also going to recommend Inkscape. But before working with this software, I would suggest that you should do yourself a favor and take some time out to watch Sara Adams' Inkscape tutorial http://www.happyfolding.com/instructions-fuse-whirlpool_spiral_cps_with_inkscape where she explains how to create a CP (crease pattern) for one of Tomoko Fuse's spiral designs. Even if you aren't interested in drawing those, she covers a lot of tools and tricks in the process that you will find useful for drawing traditional diagrams. You can thank me later.