Here is a small tool for grabbing the thumbnail image for a youtube video — something I sometimes need to do and always have to DDG or Google. There is a very helpful StackOverflow answer, which it is straightforward to implement in Javascript (have a look at the source code of the page).
Update 2018: Ed Johnson-Williams has made a site that does the same thing, but can also overlay a play button on your youtube thumbnail.
Just paste a youtube url in the box, choose your resolution and you’re away.