Re: That warm, fuzzy feeling
I use tinkerCAD to make parts on my 3d printer; occasionally, It's been used to modify other people's parts to fit my needs. (such as adding holes, or slapping a solid bit to the shape and then adding a hole to that as an additional mounting flange)
The UI is... mostly intuitive, but can be infuriating at times. But, hey, it's free, so I can't complain overmuch about it. :)
Amusingly enough, I broke my old lego set out of cold storage a month ago to prototype a frame to hold a set of small (1.5") TFT displays and rig up something that can open the lid of a wooden chest; I have this neat idea for a halloween prop and figured it'd be faster to protoype the structure in lego and then build out what I need using lumber and 3d printed parts.