One of the handiest tools in my Linux toolchest is Ffmpeg, the universal media transcoder. I'm pretty sure this fantasically useful piece of software runs in the background of every service that converts audio or video from one format to another (Google video, YouTube, Archive.org, etc). Over the years I've amassed a small collection of commands that will allow me to take media from one source and convert it for use in another - for example ripping video from a DVD or the web and then editing it and burning it back to DVD.