Mozilla Jetpack

Jetpack is a new experiment from Mozilla. In their own word:

"Jetpack is a newly formed experiment in using open Web technologies to enhance the browser, with the goal of allowing anyone who can build a Web site to participate in making the Web a better place to work, communicate and play."

Step 1: install jetpack
Step 2: reload this page
You will see a message at the top on the page inviting you to install this feature. Click on the Install button.

You can download the source here.