Conversion and Basic Editing
MeshLab Makes the Connection
When I first started looking at 3D design programs to use for CNC, I used Sketchup. For one thing, it was easy to evaluate with the free version. For another, it seemed to have the sort of drafting-oriented mentality that I have. But, it's got a file format that's foreign to almost everyone else in the world.
Before I raced off to buy the "Pro" version, which imports and saves in some standard formats, I decided to look for a free conversion tool so that I wouldn't be locked in just yet. MeshLab was the best I found, of about five programs that I tried (and more that I looked at). MeshLab is more than just a file conversion tool, though that's what I started out using it for. It has some basic mesh editing and correcting utilities built in. I found it was able to get me STL and OBJ files from my Sketchup drawings, and it was also able to correct some topological problems or do minor touch-ups to my files.
MeshLab supports an excellent array of formats, and handles them very well, retaining as much detail as the format can provide on an import. It also handles options in export well, trying to convert and save as much information as necessary. Sometimes it's hard to understand what it's doing or asking about if you're not familiar with the underlying formats, however. Normally, I found that using its defaults worked pretty well. But there were times when I got into settings and had a hard time with it.
While ZBrush now has native STL export capability, I still use MeshLab occasionally to fix problems with mesh topology. Some operations in ZBrush can cause topology to go a bit wild. I've got various work-arounds, mostly because I often don't discover that ZBrush has borked the mesh until after I've put another several hours work into it elsewhere. Sometimes built in functions in ZBrush will correct the problem. Other times they would completely mess up the mesh. In those cases, I can load the mesh into MeshLab and correct problems that ZBrush can't deal with sometimes, as an alternative to rolling back half a day's work or more in ZBrush. Closing holes is a biggie, especially with meshes imported from other programs that ZBrush doesn't "get" the way they use the particular file format.
It can also make simple modifications to the nodes and edges in a mesh, align features, and so on. It does best with low-poly meshes. But it really is a light 3D editor in addition to being a file format converter.