I have a Richard Solo 1800 portable battery charger for my iPhone (actually I now have 2) which each extend the battery life by 100%. Useful for travelling. The first one I bought has lost some oomph but I use my iPhone A LOT!
I got them from richardsolo.com. They also make a Blackberry model. There are other models, smaller and less power or heavier and more powerful.
Also zagg.com sells at least one model (ZAGGsparq) which powers USB devices. Until tomorrow, you get free earbuds with a ZAGGsparq
http://www.zagg.com/j/promo/buysparq-freebuds.php
They normally sell for $49.99 I believe.
There are probably other options out there, but those are the two I know of.