Meshmixer

Meshmixer is a free tool by Autodesk for combining and sculpting 3-D models. Sculpting-style tools like this are really easy to use, and great for lumpy organic things too. You sculpt by clicking somewhere and using the movement of the mouse to push and pull bit sof your shape. You typically start with a sphere or box, but you can import other models too.

It automatically collects usage data, so be sure to Help>About Meshmixer to turn that off if you don't want your data sent.