voronoi can be done - but it's a real pita.

I read the how-to for meshmixer and lost the will to live :-)
Ended up poking semi-random holes into the thing instead.

To answer your question - the best program to produce the moulds would be openscad.
Either download from thingiverse or another model repository - the model you want to make a mould of (or make your own) and then import into openscad where a basic difference opertion will create the mould.

Haven't got into the artistic modellig side of things yet. but hoping to in the summer. Got a book on the 123design suite of programs I need to sit down and work through.
http://www.123dapp.com/

Bear in mind that a lot of cad programs don't actually do solids. And for 3dprinting you can only print solids. So it makes sense to just learn the programs that will produce printable models.