I have been using a 3G (3rd generation) iPod for some time that would not hold its charge. I was forced to always keep it plugged in and miss the advantages of listening where ever I wanted. Then in April I ran across an article that said there are replacement do-it-yourself batteries.
Sure enough, I found one at Fry's. [I just searched Google on "ipod replacement battery," and got over 5 million results.]
It was not easy, but I did successfully change the battery. The not-so-helpful instructions within the package were line drawings, so I went on a search for better instructions. Most of what I found was not any better. Take for example the instructions from ipodbattery.com. They have a much better visual representation than my original instructions, but they never really help you understand how to remove the case and just how hard it is.
I eventually found eshop.macsales.com, and they provided a video -- once I learned how to navigate to the right video. In their example, opening the case as not as easy as shown (I think the same iPod must have been opened several times), but I eventually got it.
So, if you have an older iPod that does not hold its charge any more, try a new battery -- just prepare yourself with the struggle of opening the iPod case.