Thousands of Greasemonkey scripts are available for download out there. But its very hard to find scripts which are really useful. There are way too many scripts to perform the same task. Here I have tried to compile a list of some useful scripts for Youtube. If anybody knows of any scripts which was left out, please be kind enough to comment on it.
Automatically adds embedded videos after links to pages like: glumbert, metacafe, google, yahoo, photobucket, youtube, myspace...(and many others) so you can view the video without opening a new page.
Save video clips from YouTube, Google Video, Myspace, Metacafe, Break.com, Putfile, Dailymotion and Sevenload.
Thumbs2Links is a Greasemonkey script that gives you the ability to download large groups of videos in a fast and easy way. It works on Youtube and similar sites.
YouTube Cleaner 2.1
A simple script that removes unwanted clutter on YouTube pages. What makes it special is that it includes controls to restore them instantly.
Removes ads, allows downloading and resizing videos, displaying all comments on video page, etc. (fixed)
Mods Youtube videos, includes only the video/searchbar and enlarges the video.
No YouNoobs - Hide YouTube Comments
Hides the comments in YouTube.
YouTube comments next to videos
Puts YouTube comments in a scrollable box on the right side of the video so that you can read them without scrolling away from the clip, similar to Google Video.
Download YouTube Video and Convert with Vixy
adds "Download This Video" link and icon to Youtube , on click you will be redirected to Vixy or DirectDownload.
A script that blacks out the rest of youtube when watching a video. This script reduces distraction, allowing for maximum viewing experience.
YouTube Full Description
Automatically shows the full description on YouTube video pages.
Youtube Fullscreen Redirector
Redirects YouTube videos to the full screen version.
Turns embedded YouTube videos into clickable images that link to the YouTube video page.