This is a CAD design of the 3-Way joint shown in Matthias Wandel's cool woodworking video:
The joint fits into itself in 3 different angles (straight, and two different 90 degree bends). There is also a hole through the joint to bolt it together with a screw.
The version here is scaled to half size to reduce the cost.
The model file is also available for download if you want to print at home. I added 0.1mm clearance on the mating surfaces, so at full size there should be 0.2mm clearance - make sure your printer's tolerances are sufficient.
I printed it successfully in PLA at 50% size on a RepRap Prusa i3 with Cura, and it's a good, tight fit. I haven't printed it through Shapeways so far - feel free to try, but no guarantees regarding the fit.