Another Youtube Hack :Works with other Video Websites also

Download  video from site like Metacafe, MySpace, Break.com, and many more video-focused sites with simple bookmarklet.You can save file as .Flv or .MP4



Add All-In-One Video Bookmarklet   to Firefox.Open your video site and click bookmarklet.it will take you to download links as shown in the image above.Right click to save.
Note:This will not convert your video into other video.
Related:

Youtube Hack: Download Video With java script ..No Software or Website Needed