The Cluster HAT v2.3 does what it says on the tin. It allows you to combine a Raspberry Pi A+/B+/2/3 with up to 4 Raspberry Pi Zeros. The latter connect via a Controller Pi GPIO and are configured to use USB Gadget mode, making it easy to simulate or test small-scale cluster computing. The Cluster HAT v2.3 is designed by Pimoroni and is available to order from SB-Components for US$49.22.