This add-on exports the active Mesh object as an SVG document ready to be printed and crafted into a real-world model.
For basic testing or simple models, it is enough to run the Export Paper Model operator and pick a file name. Multiple files will be created if more pages are necessary, since some vector editors (such as Inkscape) cannot handle multipage SVGs.
Models containing N-gon faces (N > 3) may produce a distorted net and models containing doubled vertices, edges or other such ugly phenomena may cause the script crash.
The layout of the net can be customized using Seams, representing the edges to be cut. A preview of the result can be obtained by running the Make Unfoldable operator and viewing the separated islands in the 3D View (controlled in the Toolbox) or in the UV editor, if that option is enabled during unfolding. The operator will soon be renamed to Unfold.
Further details and part of the algorithms used are described in the manual page on Blender wiki.
Current version (at the time of posting) of the script for Blender 2.69:
Current version of the script
A prerelease version for Blender 2.70, including tabbed interface.
The script uses tabs for indentation because I just like them. However, if that is a problem, I can of course convert to spaces.
I posted this addon as a patch two years ago, but that one is closed now as the script was not working properly back then.