3rd Party Skinning Tools
Skinning Tools
Joe Alter's LBrush
Kickstand Stretchmesh
Discontinued in 2011 and made open source.
Versions here:
https://code.google.com/p/stretchmesh/downloads/list
Efficient elasticity for character skinning with contact and collisions (Disney)
(this isn't a tool that's available per se, but can provide some insight perhaps..)