Window Expander For YouTube is a great extension for Chrome users. With this, you can maximize the videos you watch on YouTube so that it occupies the entire window, while still allowing you to see the other tabs you opened. 

What makes it different from the full screen option on YouTube videos is that, using this won't change the size of the actual browser window. Rather, it fills the window with video and you can still see the content's URL at the very top.

This is very useful for people who have multiple monitor setups. It is also free to use, so there is no need to worry about any sort of payment.