RPi GPIO/header diagram

The Raspberry Pi comes with a .1″ header pins which can be used to interface the RPi with the outside world.

The guys at Raspberry Pi Beginners made a nice XLS file(http://goo.gl/mdVSm)  which shows the pinout:

DNC stands for Do Not Connect as these pins are reserved for future expansion.

Pin 1 is labeled as the top-left pin.

Connecting up my voltage meter, Pin1 shows a nice stable 3.3V, Pin 2 shows 4.81V, a bit low for the 5V.