MediaCoder is a free application which acts as an all in one solution for converting video and audio files from one format to another. For example it's possible to conver a Quicktime movie to a DIVX movie.

MediaCoder has a complex but reasonably easy frontend to manage conversions. Many audio and video formats are available but some of them are read only formats such as Quicktime and Windows Media Video format due to licensing issues.

If video and audio conversion are your thing the MediaCoder is worth a look.