I was quite surprised at the amount of different battery options, primarily from China and Hong Kong.
Below is a list of the things I found, I've not personally bought any of them but I have bought stuff from HK before and I had no problems.
The first I came across was a simple plugin external battery designed to plug in the bottom of the IPhone/Ipod, this one has a 1000 mAH Li-ion battery.
There is also a pass through connector on the bottom so you can connect the battery / Ipod to the PC, you also charge the battery by using the Ipod sync lead and connecting it to the PC.
There were also some variations on this that were more shaped to match the IPhone/ITouch when in use. I found this available form HK and the UK for around £11. (in Black, White and Silver)
There is also a version with a slightly lower battery capacity (800 mAh) but with a solar cell built in to allow the battery to be charged whilst in the sun.
The next one I found does two jobs, it acts as a jacket with a battery, 2200 mAh, and also a stand/dock for use on the desk.
This I only found in HK for around £15 inc. Del.
Again all the charging of the Li-ion battery is done using your existing sync cable.
The next one is just a jacket that fits around the IPhone 3G and contains a 2400 mAh Li-ion battery and also a 1.5W speaker, this allows you to listen to the audio as the jacket covers up the normal audio aperture. again this battery is charged by using the IPhone/Ipod sync lead that came with your IPhone/Ipod.This option is a bit more expensive at around £25 and can be found in HK or UK.
The last one is a leather flip case that includes a Li-ion battery within the case and a connector at the bottom.
The battery is 2100 mAh and you charge it using your sync cable, I only saw this item available from HK.
Of course there were many other types of batteries for the IPhone/Ipod including AA battery holders with the Ipod connector and all round chargers with a Li-ion battery and a kit of connectors for many different gadgets, some even have solar panels to recharge the Li-ion battery so, in theory, they should always be topped up and ready for use.
