I'm looking for a portable battery that can charge my pmp on the go via usb cable. I want it to be as small as possible.
I have a pair of Duracell PPS2 which are really small and light, but only hold some 1000 mAh. About half the capacity can be put to use. I also have a few noname external batteries from eBay, but they convinced me that in this case, it's a good idea to go with reliable brands. So I'd take a Duracell, GP or Energizer, chose the capacity as you need (up to 7 Ah from GP).
Some people also build their own packs from Li-ion cells from Dealextreme, but I'd be wary of that since li-ion batteries aren't exactly straightforward and can be really dangerous when not used properly.