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ConvertTube is a service that will let you convert and download an online video by simply providing the URL.
The options for file conversion let you convert for Windows, Macs, Flash files, audio only, iPhone, iPod, PSP or mobile. Once the conversion is complete, you can download the file to your computer and have your way with it. The name of the service and the icon imply that the service is specifically for YouTube videos, and I think that's the point. While this isn't stated anywhere on the site, I tried videos from other online sources that do allow for video download, and found that only YouTube videos will be converted for this service.
Oh, did I mention the site is for sale? You can buy it direct for just under $14,000.