I used juice defender, but I got annoyed at having to reload pages every time I opened a webpage or facebook because it had disabled the net.

It certainly did save battery though.

I now use Llama to save batteries by turning on and off wifi and bluetooth depending upon my location. - a very useful bit of software if you do repetitive tasks. like streaming music through your bluetooth car setup. play/pause on connection.

I also use it to control ringtone volume depening upon location and time. quiet at night, silent at work, loud every other time/place

Free too!