In a previous post about offshore iPhone chargers, a few astute TUAW readers mentioned Monoprice.com as a good source for chargers, connectors, and computer, TV and audio cables. In searching their site, I found a real deal.
The Monoprice Backup battery pack is the simplest of devices. When your iPhone or iPod is running low on juice, just plug this battery pack into the 30 pin connector. You can then either keep running with a fairly large dongle hanging off the bottom of your device, or wait and the battery pack will charge your device in 3.5 hours according to Monoprice (or about 2.5 hours according to my tests).
Yes, there are a lot of battery backups that do this, but how many are sold at US$14.50? You read that right, only US$14.50, and for that you get pack with a capacity of 2200 mAh.
For comparison, the capacity of the iPhone battery is 1150 mAh, so this battery should be able to charge your device twice. Due to my ignorance of the vagaries of electricity, mine winds up charging the iPhone about 1.75 times, which isn't bad at all.