Raspberry Leaf GPIO Reference
If you follow RaspberryPi.org, you've probably already come across this. But I feel the need to reiterate it here. The Raspberry Leaf is one of those ideas that seems utterly obvious in retrospect, but requires the right person at the right time to come up with it.
Simply print off this little Raspberry Pi GPIO Reference, stick it on your Pi's GPIO pins, and be forever liberated from constantly looking up the pinout on the web and counting pins.