What"s there to see at a Macworld Expo without Apple attending? Plenty, if you own an IPod, or at least have a collection of digital tunes stored on your computer--even if you don"t run the Macintosh OS. Apple chose to forgo this year"s Macworld Expo here, unhappy with the decision to move the show from New York. But many third-party developers and software application vendors have made the trip further north. Among them: Several companies offering protective and decorative IPod cases, which seem to be at every other booth. Here"s a rundown of some of the more interesting products on display that don"t require you to own a Macintosh PC.