Google Chrome Extensions gallery has now more than 1500 extensions ! Congratulations, Google chrome team, and all developers out there !!
Here is what an extension is, Google’s Own words ..
Extensions are little programs, created by developers, which add useful functionality to the browser and to the websites you visit. Some provide you with alerts and notifications, others let you easily access your favorite web services from icons next to your address bar, and there are lots more.
Now, a short video explaining how to use Extensions