USB flash drives aren't just for storing files anymore. Plugged into a laptop or desktop, thumb drives can run a range of portable apps without touching your hard drive.    USB keys aren't just for apps, of course. They provide general storage for anything and everything, from simple documents to entire digital movies. They're handy for transporting photos to a photo printer with an integrated USB port, or dragging around that giant PowerPoint slide show instead of lugging your laptop and a projector.