Battery Guru is a menu bar app that sits in your menu bar and displays advanced information about the current state of your battery.
When you are using your battery have you ever noticed that the amount of time you have until the charge runs out often drastically varies?
The reason your battery sometimes drains quickly is usually due to one of two reasons:
1) Sometimes an app you are not even using will sit in the background constantly using 100% of one of your processor cores. Having a Flash application open in one of your tabs is a common cause of this.
2) As you may know your MacBook Pro actually came with two graphics cards. One that uses little power, and one that is much more powerful but uses significantly more power as well. If you have Lion and a more recent MacBook Pro your OS will automatically switch between the two, but sometimes you will have an app or a webpage open which holds the power hungry graphics card open unnecessarily draining your battery.