Not a specific Pre accessory, but this thing is worth mentioning.

I used it today for the first time and its a beast. Heavy usage on the Pre today and the fueltank topped me off to 100% on my commute home, while still having juice left in case I needed more. I charged from about 40-50%% to 100% about 3 or 4 times today. It says 500mah output in the description but I emailed them and they said it goes up to 1000mah, which it must bc it charges the Pre VERY fast. I keep it in my bag because I take mass transit, so its good for those that commute similarly.

P.S.-It comes with a miniUSB connector, so I added a mini->microUSB adapter which works great bc it gives me some slack so I can still use my Pre. I wasted the voucher for my free connector on a microUSB connector, not realizing how short it would be. I would have been better off getting the standard USB connector and plugging in my usb cable, which is what I may end up purchasing anyway.

And oh yea, Costco sells it at a fraction of the price than anywhere else, I did the research. Costco - Callpod Fueltank® Rechargeable Dual Device Power