View YouTube videos into a dark environment, like in a cinema.

Another good extension for Chrome is, again, made for YouTube. It will make the white area from the browser darken, so you will have the sensation that you're in a cinema. I really like the feature. Try it by searching "Turn Off the Lights" in extensions (extensions homepage is here) and selecting the second result (Mac users goes here). It has a button, on the right part of the address bar.

Here's a screenshot of how it looks like: