Since I just discovered that my online video converter of choice, vConvert.net, is no longer online, here is an alternative that is nearly identical in look and operation.
Converttube.com will convert your YouTube videos to a bunch of popular formats including, Windows, Mac, Mobile, iPod/PSP/iPhone, Flash (.flv), and audio-only. All you have to do is provide a YouTube URL.
I know another:
Http://www.catchvideo.net