Mozilla Jetpack 0.1.2
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.
In short, Jetpack is an API for allowing you to write Firefox add-ons using the web technologies you already know.
As with all Labs experiments, Jetpack is an open source project and everyone is welcome to participate in its design, development and testing.
Firefox Add-on for Windows, Mac or Linux
Download: Mozilla Jetpack 0.1.2