Chrome/Firefox with Greasemonkey: If you like to watch YouTube videos without all the clutter of the side bars, userscript Resize YouTube Player can help by automatically stretching the video you're watching to fill the entire browser window. This script also automatically changes the video's resolution (where available) to a higher quality, depending on how large your window is.
Resizing the window will automatically adjust the video to fill as much space as it can. If you scroll down, you can still see all the typical YouTube content, like comments and related videos. They're still there, just tucked away so you have a distraction-free viewing experience.