Tell Me More
Right now, learning more about something you read is pretty cumbersome. You've got to open a new tab, search for whatever you wanted to look up, select a result, remember where you left off when you get back to your original page... Tell Me More makes that a whole lot easier. Learning more is as simple as selecting the text you want to look up. The moment you do, Tell Me More will show you a hovercard containing information about that from Wikipedia. If that's too intrusive, automatic lookups can be disabled. Instead, use the right-click menu and select "Tell me more...".