Does anyone know of a good free voxel editor and/or voxel-to-mesh converter?
Thermite3D is a voxel-based game engine. It isn't an editor per se. It does, however, have a list of voxel editors on its wiki here: Thermite-Recommended Voxel Editors.
Of those, Sproxel, Voxel, and QBlock are all free. Paint3d and Everygraph (Voxel3d) have trial versions, and one not on the list, Qubicle Constructor, has a crippled trial as well.
I wrote one for my Voxel Roguelike. You can get it here: link
If you need some info on the file format, just post a question on the blog and I can help you out.
There's also Thermite3D. I've looked at it a little but haven't actually used it. The forum community is very friendly and helpful, however.
You can try this (highly experimental)
You could try this one http://dmytry.pandromeda.com/voxelworld/, though I think it's not being maintained anymore (still, it's open-source, should you want to mess with the code).
This is the one I use, by Eric Haines: http://www.cs.princeton.edu/~min/binvox/