StayFocusd Chrome Extension limits the time that you can spend online

StayFocusd is a productivity extension for
Google Chrome that helps you stay focused
on work by restricting the amount of time
you can spend on time-wasting websites.
Once your allotted time has been used up,
the sites you have blocked will be inaccessible
for the rest of the day.
It is highly configurable, allowing you to
block or allow entire sites, specific subdomains,
specific paths, specific pages, even specific
in-page content (videos, games, images, forms, etc).

You must use Google Chrome as your browser to use StayFocused. This could be quite a useful tool for both students and adults alike.