The ubiquitous numeric display made from 7 LEDs arranged in a figure 8. Usually they also have a decimal point. It's often annoying to figure out which pins map to which LEDs. Then, once you know the LED pin-mapping you must figure out the mapping from the individual LEDs to the numeric digits. We have a few tutorials with sample code that have this all figured out for Sparkfun's big 7 Segment LED
to help you jump right in.