Is there a simple way (on the command line - I want to write a script which compresses all videos in a folder) to reduce the filesize of a video (almost) without quality loss? Is there a method which works equally well for different video format (mp4, flv, m4v, mpg, mov, avi)?

I should mention that most of the videos I would like to compress are downloaded web-videos (mp4, flv), so it's not clear if there is much room for further compression.