I know someone recently made an app that will flash the LED when the phone is fully charged but people like myself, who always leave their screen down, would not be able to benefit from it. I found a nice little app that will play a tone when you phone reaches a full charge. You can find the link to it below.
Battery Tune
Edit: I've been using it for a few days and it works fine. You need to have your media volume turned up to hear the charged tone.
Battery Tune
Edit: I've been using it for a few days and it works fine. You need to have your media volume turned up to hear the charged tone.