PhotoScore is a music scanning program - the musical equivalent of a text OCR program. It is also able to convert PDF files to editable and playable notation.

What do people use PhotoScore for?

Accompaniment and guidance when practicing instruments
Transposing (for example to bring a vocal part into range)
Scanning music for use in sequencers or other editing programs; for re-arranging or extracting parts
Scanning handwritten music
Creating MIDI files to put on web pages for others to hear
Just for fun! With PhotoScore you can scan in your favorite song, have it played back to you, change the instruments and so on!