iPhone 3GS Replacement Battery
The 3GS battery is rated to last about 500 cycles. If you start each day with a fully charged phone and run it down to 0 each night, the battery will be below 80% effectiveness in less than two years. Luckily for all of us, Apple comes out with a brand new phone every two years! How considerate.
If for some reason you're happy with what you got, this simple replacement will keep your iPhone 3GS running for another two years at least.