Proto Screw Shield from Sparkfun. Although it's a very nice piece, and easily assembled, I received one from Adafruit today that I believe to be a bit better. First of all, It's REV 3 compliant, which the Sparkfun piece was not. Second, it also came with the pass through extended headers for the traditional ICSP block. Construction time and ease was essentially identical. Sparkfun still has the better shipping rates, so if you order from Adafruit, you better have a large order. I get charged $15 S&H for a $3 transistor or $100 worth of boards.