YouTube can be a cluttered and distracting place if you want to just watch a video. It's good to know that there are simple extensions for your browser to improve YouTube.

Naked YouTube


Browser: Safari 5
The Good: If you are looking to blackout the rest of YouTube while watching a video, this extension works beautifully. It completely blacks out all of YouTube's interface, allowing the video to stand out.
The Bad: After the video finishes, there is absolutely no way to return to the video's original page. That means you can't search without going back to the YouTube main page. It also means you can't see any related videos on the side, and any other videos in the playlist. Additionally, it removes the ability to stop autoplaying, so unless you switch pages, you're caught in an infinite loop of YouTube videos.
Website

YouTube Enhancer


Browsers: Firefox 3.6 and up (with the Greasemonkey Extension), Opera, Google Chrome
The Good: This is one of the most full-featured YouTube modifications around. It adds a plethora of features, including download options, quality selection, and a blackout option. The "lights off" feature, located in the upper right-hand corner, blacks out the whole page, save for the options below the player. Even better, you can click anywhere outside the video to return to the normal YouTube page.
The Bad: There are a ton of features, and for the casual YouTube user, they're unnecessary. The script's interface isn't so nice either, and definitely stands out against the polished YouTube appearance. If you are just looking to temporarily eliminate YouTube distractions, this userscript is definitely overkill.
Website

Turn Off the Lights


Browser: Google Chrome
The Good: This extension does one thing, and it does it well. It places a small lamp button in the URL bar, and with one click it darkens the whole page. After the video finishes, just another click returns to the normal page. There are a ton of options to customize how you want the add-on to work, such as darkening the page automatically after clicking the "Play" button, and selecting certain buttons to remain visible during the blackout.
The Bad: There are issues with the icon appearing in the URL bar on macs, but quickly disabling and reenabling the extension fixes the problem.