paper: A Content-based System for Music Recommendation and Visualization of User Preferences Working on Semantic Notions
essentia https://github.com/MTG/essentia C++ library of algorithms to extract features from audio files, including Python bindings.
gaia https://github.com/MTG/gaia C++ library to apply similarity measures and classifications on the results of audio analysis, including Python bindings. Together with Essentia it can be used to compute high-level descriptions of music.
freesound The freesound.org website
and many other interesting things