Multiplayer Piano is a free Chrome app that lets you play piano online on your Chrome browser. You can also play with multiple players at the same time.

Search by Image (by Google) is a free extension available for Google Chrome that allows you to start Google search using any image available on web.

Google Similar Pages (by Google) is an extension for Chrome that helps you to get similar web pages to the page that you’re currently viewing.

Click&Clean is a Google Chrome extension that lets you clear browsing history, typed URLs, Cookies, download history, and much more. Add it for free.

Yahoo Mail Widget is a free Google Chrome extension that lets you check Yahoo! Mail in Chrome browser. Check it out here!

Feedly+ is a Google Chrome extension available free of cost that lets you quickly access desired news sites and blogs. Read articles easily and share them.

Heap Note is a free Google Chrome app that lets you create and organize tasks, schedules, and notes at one place. Check it out here!

Resolution Test is a free extension available for Google Chrome that helps you to test web pages on different resolutions easily.

Stealthy is a free Google Chrome extension that lets you browse anonymously and access blocked websites and online services easily.

Here is the official Firefox OS simulator addon for Firefox browser by Mozilla.