Juice defender is also pretty great.
The best way to make the battery last though is not use the phone. You'll actually get several days in standby mode on airplane mode. It's great!
If it weren't for physics and law enforcement, I'd be unstoppable.
HTC One - Current
Motorola RAZR Maxx
HTC Droid Incredible 2
Samsung Galaxy Fascinate
HTC Google Nexus One