If you don't spend all your free time with your nose buried in a computing magazine, you could be forgiven for not knowing. But USB gadgets (known as "key rings") are hot. These are miniature devicestypically digital cameras or MP3 playersthat plug directly into your USB port for easy up- and downloading on the road. They're cute and trendy and getting more useful all the time. This summer Philips introduced two models for Mac or PC, priced from $99 to $149. The company's MP3 Key Ring comes in 64-MB and 128-MB sizesjust right for a short music playlist. Some versions...