Utility that displays the details of all bookmarks stored in Chrome and Firefox Web browsers.
Open Youtube videos on the same page you are on.
Dooble, the scientific browser.

Don't have the patience to wait and type unreadable codes on sites like Rapidshare and Megaupload?
Firefox extension that displays relevant Wikipedia articles alongside your Google search results.

Add-on for Firefox that allows you to post to Twitter through the bar in the browser.