It is not as common as it once was, but every so often a student comes in with a file created at home in a format that your applications cannot read. I use to have a collection of file conversion utilities to handle these situations, but today I don't have any easily at hand. Zamzar is an online tool which takes any file up to 100 megs in size and turns it one of many other possible formats. Unfortunately, it does not handle my Real video files, but it gets most common formats.
Zamzar can probably be of most use with multimedia files. Unless you are creating all your content, you're probably going to be working with multiple formats. It's a real pain in the neck to have a sound track for your video only to find that it's an invalid format.