The first is the XOrduino, a stripped down low-cost Arduino-compatible board that plugs right into the XO's USB ports. But wait, there's more: it's also compatible with the Scratch Sensor Board, so you can use this device to control Scratch (and Turtle Art, once Firmata is ported). It should be compatible with the Arduino IDE and all Arduino Leonardo-compatible shields.
The board uses mostly through-hole parts, with one exception, and there are only 20 required components for the basic Arduino functionality, costing about $5 (from digikey, quantity 100). It is reasonable for local labor or even older kids to assemble by hand.
It's open hardware: Eagle design files are on github (schematic PDF, pcb PDF). I expect to have a small number of boards in a few weeks; let me know if you'd like one in exchange for help with hardware and software bring-up. Schematic and layout review also appreciated (I did the PCB routing late at night under time pressure leaning heavily on autoroute, it's certainly not the prettiest). And feedback from Arduino and Arduino shield hackers would also be welcome.
If $5 per student is too much money, there's also the XO Stick, my second board. It's based on the AVR Stick using the ATtiny85 processor and costs only $1/student. It's not quite as user-friendly as the Arduino-compatible board, but it can also be used to teach simple lessons in embedded electronics. For $0.12 more you can populate an ATtiny261A (though a '461 or '861 would be better) and get 13 I/O ports; this variant should be powerful enough to program other XO Sticks and perform XO maintenance tasks (accessing the serial console, debricking a laptop via SPI flash). The XO Stick is even easier for a kid to assemble themself: only 8 required components, all through-hole. (Sadly, my desire to shave every penny off the cost of this design meant that I couldn't use some of the symmetry tricks I invented for a 2012 Mystery Hunt puzzle to make the circuit impossible to assemble incorrectly.)
Same deal as the XOrduino: design files on github (schematic PDF, pcb PDF); I expect to have a few boards available to people who want to help make some software for them. Schematic and layout review is also appreciated!