BpmDj - open source Linux.
Java Beat Detector - description of an algorithm and source files.
Beat Detection Algorithms, by Frederic Patin - statistical approach of beat detectin and filtering approach of rhythm extraction.
Ess sound library - sound library that allows sound sample data to be loaded or streamed (AIFF, WAVE, AU, MP3), generated in real-time (sine, square, triangle and sawtooth waves, white and pink noise), manipulated (raw or via built-in filters), saved (AIFF, WAVE), analyzed (FFT) or simply played back.
Aubio - a tool designed for the extraction of annotations from audio signals.
Echonest API - From the site: "Hundreds of music applications tap into The Echo Nest API for access to billions of data points about music from leading media companies (MTV, the BBC, MOG and others) to award-winning mobile applications (Discovr, Music Hunter, Pocket Hipster)."
Vamp Plugins - Vamp is an audio processing plugin system for plugins that extract descriptive information from audio data — typically referred to as audio analysis plugins or audio feature extraction plugins.