Youtubescreenshot is a free web app which allows you to easily take good screenshots from any Youtube video. Once you have provided the URL of the video clip from which you want to extract one or more screenshots, you can either move the playhead of the video to find your preferred frame to capture an image, or rely on the tool automatic identification feature. In addition Youtubescreenshot serves a set of four thumbnail screenshots (at 480x360 and at 120x90).
My comment: Very simple and easy to use, Youtubescreenshot delivers reliably what it promises.
Try it out now: http://www.youtubescreenshot.com/