Google Sketchup is a 3d CAD software, which is really intuitive to use, once you got the hang of it.
It's not free, but there is a gratis version that is quite powerful. It is also easily extensible using the ruby scripting API.
The free unfold script can flatten out 3d objects for paper crafting, but it does not layout the shapes, nor does it add latches or seams.
Waybe is a commercial extension that creates plug-n-play (or rather cut-n-glue) print-outs for paper crafting
Pepakura Designer can also read Google Sketchup file formats.