Clickberry Interactive Editor is a unique video editor that allows you to create interactive videos for Facebook, and extensions for Google Chrome and Firefox are available. In short, you just find the video, add an interactive feature, highlight it and upload to Facebook.
Google Dictionary by Google is one of the most useful extensions for its Chrome web browser. Once you install the extension, you can simply double click a word in Chrome and the Dictionary will pop up with a definition of the word. Not only is it easy to use but it also has support for multiple languages.