TopMod3d is a free, open source, topological mesh modeling system that allows users to create high genus 2-manifold (watertight) meshes. It runs on Windows and OSX.
To me, topological modeling feels weird, but after playing around with it for a little I managed to get some nice results. As an added bonus most models will be readily printable by Shapeways (after conversion to X3D/Collada, still).
Here are some videos that are interesting to watch:
‘Twisted Cube Handles’
(This interface version is outdated, but the way of working is still the same)
Some nice models made in TopMod3D