pHash -- the open source perceptual hash library

Potential applications include copyright protection, similarity search for media files, or even digital forensics. For example, YouTube could maintain a database of hashes that have been submitted by the major movie producers of movies to which they hold the copyright. If a user then uploads the same video to YouTube, the hash will be almost identical, and it can be flagged as a possible copyright violation. The audio hash could be used to automatically tag MP3 files with proper ID3 information, while the text hash could be used for plagiarism detection.