Click here to visit the Top Mod website and Download.
TopMod3d is a free, open source, portable, platform independent topological mesh modeling system that allows users to create high genus 2-manifold meshes.
While a little buggy and (sometimes) sluggish, TopMod has some very unique tools for creating complex meshes. You begin with a primitive shape (a cube, for example) and then manipulate it to create more intricate, continuous meshes.
The program has many subdivision and remeshing options although I wish it featured a sub-d 'preview' toggle so I could get a better idea of what the smooth result looks like before committing to a permanent operation.
Regardless of its flaws... there is still much fun to be had with TopMod...