Adafruit Super Capacitor - 2.5V 630 Farad
When I was studying power supply electronics in school, the instructor always talked about adding BFC's to the circuit - Big Freakin' Capacitors. This here is UHFC - Unbelievably Huge Freaking Capacitor. If you have a super-cap project that needs up to 630 Farads of capacitance, check it out. Like most super-caps it has a 2.5V-max rated voltage, and remember that unlike a voltage-output battery the voltage drops immediately as it discharges.
You can use this as a stand-in for a battery, its huge but can charge fast and doesn't degrade like NiMH or Li-Ion/LiPoly batteries. We like to refer to this MAXIM super-cap calculator to figure out how long we can run a project.
Super capacitors are powerful beasts: the ultra low ESR means that shorting the outputs will create a big spark! Care must be used when working with charged super-caps, these are only for experts who are comfortable with high-current-capable power supplies.