Adafruit GPIO Reference Card for Raspberry Pi Model B+
This board makes connecting wires to the Raspberry Pi GPIO incredibly easy. It's a reference board that makes it really easy to see if you're connecting your wires correctly.
It's only 0.8mm thick, idiot proof (you can't put it on backwards...), and makes a great keychain for the Pi enthusiast. Simply slide it onto the 2x20 GPIO header, then you can plug in premium jumper wires with female sockets on top as shown.
This version works with the Raspberry Pi Model B+.