Most of you will already have seen this (I guess): one of my very first designs.
It's a 3D adaptation of the old 'impossible dovetail':http://www.youtube.com/watch?v=d9xwgGoMqtk
OK, it's not much of a puzzle. Although a lot of people - not familiar with puzzles - struggle with it for some 15 minutes (when the disassembly was done out of view)... before realising that they need to make two halves. Otherwise they'll end up with a single block that won't go in, no matter which one they select to be 'the last'.
If anyone of you want to make a copy for himself... well, good luck!