I was introduced to these by a coworker, and while I haven’t tried them myself, he uses them on a daily basis to power his wireless mouse. They are batteries which are recharged using a USB port. Sure, they might not be up to USB specification-snuff (rules are meant to be broken, no?), but they’re cool nonetheless. And in my never-ending quest to carry a single power cord for all of my various gadgets, this is a huge leap forward.