COM-00746 Sparkfun Super Capacitor - 10F/2.5V
Yes you read that correctly - 10Farad capacitor. This small cap can be charged up and then slowly dissapated running an entire system for hours. Combine two in series for 5F/5V. Do not over voltage or reverse polarize these capacitors.
Documents: 10F Super Cap Datasheet
Dimensions: 13x33.5mm