dvdvideosoft.com and vixy.net (update: now gone)
Here’s the latter, a free service at the moment, converting the Amy Winehouse Rehab video so that I can listen to it and watch it on my video iPod. It’s not fast but it works. Download size was 6.88Mb.
YouTube doesn’t want people doing this, so this won’t last unless Apple and Google agree a business model that sees the iPod and YouTube getting hitched.