I recently received a cardboard bird box as a gift and decided to put it up this weekend past. It comes 'flat-packed' and is very easily assembled by simply folding it together and pushing tabs into place. No glue or scissors required. Finally a wooden dowel pushes through the middle to act as a perch.
Apparently it's waterproof though it's slightly hard to believe. Perhaps a bigger problem is that it's so light I was worried it would simply blow away. It has a hole in the back to hang off a nail/screw, but I added some garden wire to try to hold in place more securely on my old apple tree. It will be interesting to see whether it finds favour as a home for birds. I will report back in due course.
Update, 22 July 2012: and report back I have.