I'd like to customize and modify some parts on Thingiverse, beyond just simple scaling for 3D printing.
I've been looking for some tools that convert the STL files into something that is easily edited, but so far all that I've found are really buggy and crash frequently as soon as one loads a reasonably complex model.
Are there any free open source software tools that people can recommend that handle STL importing and editing? If not FOSS, what about just "free or nearly free for hobbyists, ed, non-commercial?
There is no argument that the most successful FOSS 3D editor is Blender. It can both import and export STL files and export to STL or many other formats (DAE, OBJ, 3DS, FBX, DXF, WRL) so you can both use it to edit STL models as well as just use it as a STL converter. https://www.blender.org/