What are some good FOSS or free tools for editing STL files for 3D Printing? 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?
You can use OpenSCAD. It's a script based powerful CAD open source software under GPL. 
On Thingiverse, a lot of things are made with this CAD software (the Customizer flagged ones) and you can just download their source code (.scad) and directly edit them !

You can also import existing STL and edit them like they were a primitive shape like a cube. You can then interact with them by doing binary operations, adding parts, etc. 

It is hard to start with if you have never coded, but that's worth it :


  OpenSCAD is a software for creating solid 3D CAD models. It is free software and available for Linux/UNIX, Windows and Mac OS X. Unlike most free software for creating 3D models (such as Blender) it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects. Thus it might be the application you are looking for when you are planning to create 3D models of machine parts but pretty sure is not what you are looking for when you are more interested in creating computer-animated movies.