I love designing, making, solving and collecting interlocking 3D puzzles. I have lots in my microcubology shop on Shapeways site but most of them will not print well on current RepRap machines because of the need for tight tolerances and support material for overhanging blocks.
I discovered this brand new puzzle here http://puzzlewillbeplayed.com/553Ring/BisectCube/ at the excellent 'Puzzle Will Be Played' website and immediately realised that it was printable on my new eMaker Huxley machine. It took three goes to get the tolerances right but this version prints just right for me. I have based the puzzle on 10 mm cubes and used a 0.2 mm offset on every face.
Print the three parts and then assemble them into a bisected cube, as shown in the photo. A very nice little puzzle challenge as well as a good test of printing accuracy.
The STL file was exported directly from my favourite puzzle design software, Burr Tools, which allows offset to be specified prior to exporting.