Raspberry Pi: Combine a Raspberry Pi with up to 4 Raspberry Pi Zeros for less than US$50 with the Cluster HAT
The Cluster HAT v2.3, as its name implies, brings cluster computing to the Raspberry Pi. Specifically, it allows you to connect up to 4 Raspberry Pi Zeros to a Raspberry Pi A+/B+/2/3. 8086 Consultancy, the creators of the Cluster HAT, states that it will work with any mix of the Pi Zero 1.2, Pi Zero 1.3 or Pi Zero W. The HAT also comes with standoffs, screws and a short USB Type-A to micro USB cable.
Please note that the Cluster HAT does not come with a Raspberry Pi or Pi Zero. These must be purchased separately. Pimoroni provides assembly and control instructions on its product page, along with 3 ways of setting up the Cluster HAT, which it separates into simple, intermediate and manual. You can find the set-up instructions here.
The Cluster HAT uses USB Gadget Mode either via Ethernet or Serial to get the Raspberry Pi Zeros communicating with their host Raspberry Pi. Pimoroni also stresses that you will need at least a 2A power supply, or 2.5A if you are using a Raspberry Pi 3. Presumably, you will need to invest in a higher amp power supply if you plan to use the Cluster HAT with the newly released Raspberry Pi 4. You will also need at least an 8 GB microSD card for each Raspberry Pi Zero and the host Raspberry Pi. You can find the full assembly instructions here.
The Cluster HAT is currently out of stock on Pimoroni. It can be ordered on SB-Components for US$49.22 though, from where it ships worldwide.