Meshmixer is a 3d modeling and design program, but unlike others it is designed for manipulating 3d models which have already been created. It can adjust and re-size models, stretch or skew specific aspects of models, or even incorporate multiple models into a single object for 3d printing.
Meshmizer is free software provided through the AutoDesk 123D collection, anyone can download and install it for free on their personal computer.
Install Link: http://www.123dapp.com/meshmixer#download
There are a number of different user tutorials provided at http://meshmixer.com/help/index.html.