The Belkin Backup Battery Pack ($49.95, direct but available for less online and in special accessory package) is the one iPod accessory that's always in my briefcase. This small case, which attaches to the iPod with suction cups, encloses four "AA" alkaline batteries and will keep an iPod running for up to 15 hours, even if the internal battery is drained.

In addition, the Battery Pack incorporates a charge-level indicator, so you can tell ahead of time when your batteries are running low, and never be caught without your tunes again.

This isn't a new product, but I keep running into people who ask me about mine and where to get one, so I thought I'd post it to Gearlog as well.