AmazingMIDI

AmazingMIDI automatically transcribes music, converting WAV files into MIDI files. It can recognize single-instrument polyphonic music.

It's a powerful tool to help you to transcribe music, to practice musical instruments and to make MIDI files, and so on.
AmazingMIDI creates an Output File (.mid) from an Input File (.wav) that contains musical data, and a Tone File (.wav) that consists of monotone data. AmazingMIDI analyzes the Input File, assuming that every sound in the file is played with the same tone color as the one in the Tone File. As a result, even if the music contains several different instruments, AmazingMIDI writes down all detected notes as a single-instrument music.
The quality of music recognition depends on the tone color of the music. AmazingMIDI is suitable for analyzing the precise-frequency attenuating sounds like piano or guitar. The drum sound becomes only noise.
Currently, automatic music transcription has no substantial solution in general, but with some restrictions of the domain, it can be both possible and useful. AmazingMIDI is one of these systems, and is still in its infancy. However, again, it's a powerful chord analyzer.