Last week I posted a note from OWC on the new 2100mAh iPod battery for 1st and 2nd Generation iPods ($39.95 w/tool set and installation guide. If you order an iPod battery you can also get the NewerTech RoadTrip FM Transmitter device for only $9.99 by using coupon code 'HIGHROAD' when you checkout.)
I own a first model iPod (5GB) that I bought about 2 months after they were first introduced years ago that still had the original battery. After years of use and many charges, the OEM battery had about 4 hours of runtime in a test of continuous running (no button presses) instead of the 8-9 hours when it was new. (Runtime would typically be less in normal use changing settings, songs, etc. frequently, but the constant run test was used since it's easily duplicated for tests with the new battery.)

Installing the new battery was easy (took about 5 minutes or so) using the printed guide and tools included with the battery. (I had taken some pictures for posting a page on the install, but had forgotten you can just download OWC's install guide PDF doc for a complete guide w/phoos.) After charging the 2100mAh battery overnight, I ran the same runtime test (no button presses) and the 2100mAh battery ran for 21 hours and 3 minutes. Quite an improvement and really added new life to this old iPod. I had considered trying to sell the iPod before but figured it wasn't worth anything with the original battery, but now I'm going to hang onto it for awhile. This was a great 'upgrade' for this iPod and the cost was well worth it - definitely the most bang for the buck I've seen. (And even if I do decide to sell it later the cost of the battery was money well spent.)
I'd basically given up on using my iPod 5GB before the swap due to the old/original battery - although it ran 4hrs on the continuous play test, in typical use it seemed shorter and within a few days of no use, the battery was dead usually. Frustrating. Now it's like having a new iPod again (only better, with amazing runtime).
This really is one of those rare products that has a huge price/perf. ratio for those of us with older iPods.