I also recommend battery left, I've had it for a few days, and while it's definitely not completely synchronized yet, it's still great and will only get better. And it's free
Another vote for battery left. It's perfect for monitoring this. Shows you time remaining and the percent to 1 digit. You click on it and it's easy to get a break down of your usage.
Now if I could just replace the task bar battery with this one....Saw someone with something similar but believe you have to be rooted.