My stackable blades (boards) are based on a 1"x1" (25x25mm) size and stack vertically. They can also plug into some Pluggable Blades (such as the BaseBlade1) permitting a combination of both vertically stackable boards and horizontal pluggable boards. Many multiple processor solutions are possible.
While not specifically designed as a stackable, the USB module can be connected along the top edge (5V, P7..P0, Gnd) to permit the prop to drive resistors to program a 2 wire connection to a USB connector. This can be used to connect a PS2 keyboard or mouse, an I2C device, a serial device, or many simple 2 wire TTL devices capable of the being driven by propeller software. In particular, this pcb was designed to utilise software written by BradC and Micah to connect to USB masters, such as a PC or Laptop, or USB slaves such as Bluetooth etc. A USB-A or USB-B connector can be fitted to this pcb. The pcb has a single resistor on each Propeller I/O. Each pair P0 & P1, P2 & P3, etc connects via separate resistors to the two IO pins on the USB connector. No other logic is onboard.