MooMooEncode will change text files into the form Moo MoO mOO MOo, and so on. If you like, the Mac version can give your text a cow-ish accent, as well!
Or, if you just can't wait, why not try the World Wide Web Bovinifier?
If you are getting the program for the Mac, and aren't running System 7.5
yet, you may want to get a copy of Apple's Thread
Manager extension. This will allow you to process many files at the
same time, each with its own cute little animated cow.