HandBrake -a tool for video conversion
Free and Open Source
HandBrake is a tool for converting video from nearly any format to a selection of modern, widely supported codecs. For example you can convert a .mov file to a .mp4 file and significantly reduce the file size at the same time.
Reasons you'll love HandBrake: Convert video from nearly any format; Free and Open Source; Multi-Platform (Windows, Mac and Linux).