There was one game (I think one of the really old Unreal Tournament games) that had all the sound files as .wav files, which made the game huge. Many people were still on dialup at the time, so pirating it would have taken over a week to download it.

Well, one pirate group compressed all the .wav files into MP3s and wrote a quick batch script to decompress them all back into the .wav's. Turned a 700 megabyte download into a little under 100 megabytes.