I usually carry a spare set of AAA batteries in my gadget bag just in case my Bluetooth keyboard suddenly loses juice. If you’ve got devices that need batteries, you might want to check out the Apex battery charger that Popgadget popped on us earlier in the week.
You can re-charge up to four AA or AAA batteries in roughly four hours using the USB port on your computer; a feature I like. Why not leverage the power capabilities of a USB port when on AC power? The small recharging case goes for about $12 over at ThinkGeek; rechargeable batteries not included.