Be more green and save energy by getting notifications every time you leave your computer idle for too long.
Be more green and train yourself to save energy by not leaving the computer idle.
When the computer spends more than 10 minutes idle, the blue button changes to a red button with a small text showing the number minutes without any use.
After a sign of activity the notification stays for 10 seconds and the button returns to its original blue state.
If you leave the computer running and return a few minutes later, the icon will tell you how many minutes of energy were wasted, making you feel filthy and guilty... hopefully encouraging you to sleep the computer the next time, and the next time, until it becomes a habit.
Note: Chrome stopped supporting the technology that allowed the extension to suspend or send the computer to sleep mode directly from the browser, so that feature won't be available from now on (Sep. 2014).