Another option is the even smaller "open source hardware" Digispark with USB on board, arduino enabled, supporting many shields (grove shields etc): http://www.

Another option is the even smaller "open source hardware" Digispark with USB on board, arduino enabled, supporting many shields (grove shields etc): http://www.crowdsupply.com/digistump/digispark-mini-usb-arduino http://www.crowdsupply.com/digistump/digispark-mini-usb-ardu...