Juice Defender - >3 days battery life
I installed Juice Defender (free version) and shockingly I am now getting at least 50% more battery life (I normally get around 1.5 days out of my SGS2).
I havent changed my usage style (games around 30 mins a day, WIFI/Data around 1hr a day, etc).
I originally tried Juice Defender back in November last year on my HTC Desire Z. It didnt seem to make much difference back then. But obviously it has improved vastly since then!