In the same vein as my Flickr Link Preview script, I've created preview Greasemonkey script for links to YouTube videos. Since YouTube provides three thumbnails for every video, it shows up as a little slideshow of the video's three thumbnails. It's pretty handy. Never be rickrolled again!
Watch the video screencast below to better understand exactly how it works. Here are a fewvideolinks for testing with once you get your plugin installed.
That's it! Now when you see a link that might be to a YouTube video, simply hover over it and a preview will pop up.
This video is also available on Flickr, YouTube, Metacafe, Google, Break, Blip.tv, Veoh, Imeem, Sclipo, Viddler and 5min.