Solvespace (for gui based cad) or openscad (for script based cad). Both are free, open source and quite simple and straightforward (at least to me) with their interface and how they work. I personally use openscad for most of my modeling, unless it is something more 'artsy' and is easier to draw as a vector graphic than plot as a series of coordinates (a 'low poly' carabiner is as close to 'artsy' as i get). That and for testing constraints/linkages and double checking if i've implemented the constraints right in ope scad.