Monster iPod Garage

Fourth-generation iPod users need not apply, but there's a battery out that will increase the playtime of your iPod.

After nearly three years of almost daily use, my trusty old iPod was starting to give up the ghost. But thanks to a new install-it-yourself battery, it has a new lease on life -- and it's even better than new.

One interesting thing that the article mentions is that a firmware update with a drive wipe also brought an older iPod's battery back to life. Prior to the upgrade, it got two or three hours of playtime. After, it was up to 8.5 hours.
Wired News: Pumping Up the Power of the IPod