Lets take a look at the default Youtube screen that appears when a user without the script opens a video on the portal.

And now the same page after the script has been installed in Firefox:

The script changes a lot directly on the screen. The video gets resized automatically to fit the screen. Ads and other unwanted elements on the page are removed including watching now, character counter or video annotations. Options to download the video in flv, mp4 format are available directly beneath the video. A click on the video title will also initiate a download of the video directly from Youtube.

The configuration options provided by the script contain several interesting features as well. The user can change the design of the Youtube website drastically. It is possible to remove additional elements from the Youtube page, display none or up to 500 comments on a page, prevent autoplay or autodownload, bypass the age verification (logged in users only) and select the default video format and flash player quality.

Firefox users can download the userscript here while Opera users have to go here instead.

YousableTubeFix is a great userscript for Youtube addicts and users who want Youtube to automatically use a specific video quality and page layout.