Audiveris translates sheet music to MIDI files and notation, but is not for the average user. For those who don't mind installing Java, a DOS box in the background, and jumping through a few hoops, it's a fascinating window into the process or musical OCR with the program performs step-by-step. When it works, it works rather well, though you'll need to find the pay Maestro font (which is installed by many notation editors) on your own if you want to view the results. If you don't, you can still save the resulting files as MIDI or MusicXML.
--Jon L. Jacobi