U-Charge looks like an AC adapter with a special charger tail that snaps onto the contacts of Apple notebook batteries to charge them outside of the host machine. The benefit is that it allows you to keep a backup battery fully charged and ready without needing to purchase a dedicated, bulky charger.
You can charge all 10.8 volt Apple batteries produced since 1999 with the U-Charge, including batteries for the iBook, PowerBook, MacBook and MacBook Pro. FastMac will also be offering a separate 14.4 volt U-Charge that works with the higher voltage requirements of the iBook clamshell, PowerBook G4 Titanium, and iBook G3/G4 14-inch.
They expect to begin shipping orders in about two weeks and a full list of supported batteries is posted on the FastMac ordering page.