I have been using this application for a long long time now. It is called Beyond Compare. It does directory and file comparisons. I won’t delve into all the features, but do check it out if you are in the market for such a tool. There are plugins to support all sorts of file types like MP3 (id3 tag info), images, etc.
Via Stever from way back in my ZEFER days.
Update: Forgot URL: http://www.scootersoftware.com/