I'm not too sure why I made this, but I did.
It takes any 3d model (well, in obj format), voxelizes it, and turns it into minecraft blocks (attempts to colour match).
Looks a bit odd, but I'll make it so I can define which blocks to use. I'm thinking about also making a quick 'painter' for it, so you can very quickly airbrush colours in.
Here's a "live" example ("live", because it takes about a minute to actually process. looks broken..!)
http://www.mikeefranklin.co.uk/3d/gob.html
I'm sure there's probably tools out there that already do it - binvox kinda does, but not very well. I'll probably look to converting it to a minecraft friendly format next.