What are some good FOSS or free tools for editing STL files for 3D Printing?
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.