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.
This is especially impressive when compared to some of the more elegant competition, like the Mophie juice pack air with a capacity of 1200 mAh and a selling price of US$79, or its larger sibling the Mophie juice pack holding 1800 mAh and selling for US$99. The Monoprice battery backup gives you more power for a whole lot less money.
True, the Mophie cradles the iPhone while the Monoprice just hangs off the bottom, but the Monoprice works with most iPods along with the iPhone and iPod touch. I plugged it into my 80 Gig iPod Classic and it worked just fine. The connector is longer than most, so I didn't have to remove the case on my Classic to charge it up. You should note that since the battery pack is the width of an iPhone, if your iPod has the 30 pin connector on the same side as the headphone jack, you won't be able to charge and listen at the same time since the headphone jack will be covered while charging.
The Monoprice battery backup has a female 30 pin connector at the bottom for charging, which is quite convenient. When charging or discharging the battery backup flashes 4 lights that provide an indication of how much charge is left, similar to the battery displays on MacBooks. When not in use, pressing the right side of the battery display lights up the cell indicators and shows you the remaining capacity.
I really recommend this inexpensive accessory and now keep a couple of them in my computer bag. Monoprice has reasonable shipping prices and a sliding price scale. Buying 2-9 at a time brings the price down to $14.08.