Raspberry Pi Stacking Tower
This is a minimal yet relatively secure way to stack your Raspberry Pi's vertically. This is a universal B design in that it works with the original B, the 2 B, and presumably the B+ (I don't have a B+). The design goals were something that was quick, minimal, and held the Pi in place even when cable tension wants to pull it away.
3D model and more available at [ thingiverse.com ].