BatteryBar shows you in a bar on your Taskbar exactly how much time you have left on your battery, and how much battery life you've already used. You can have it display battery life either as a percentage of battery life remaining, or how much time you have remaining.
In addition, hover your mouse over BatteryBar, and you'll see much more information, including total battery capacity, discharge rate, battery wear, and how much total capacity your batter has in terms of time per full charge.
When you first run BatteryBar, you won't see it. You'll need to configure your Taskbar to display it. Right-click the Taskbar, select Toolbars, and then select Taskbar. When you do that, BatteryBar will appear on your Taskbar.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.