i go on long trips that I might not have a wall plug in (bicycle touring). I have two of these bad boys:
they power the nook color and all of my phones no problem. each battery has 2 usb ports to charge and one cable to charge my laptop. It's a lot of money, but I can charge my nook color 5 times over with one of these.
Two more options just became available:
I JUST ordered the 2400mah this week... But some consolation, by a pure mah/dollar point of view, the 2400mah is the best: