BeatBlender is a step sequencer that triggers samples from your music library. There are 4-channels (kick, snare, hi-hat, and cymbal) and BeatBlender searches for these timbres in your songs. I downloaded lots of sounds from Freesound.org, ran them through Echo Nest's analyzer, and built statistical models of the timbre and loudness for each channel (ie there's a model for what a kick sounds like). Then your songs are analyzed with the same analyzer and segments are found that match these models. There's rudimentary control over the sequence, tempo, and threshold for matching the model.
Python
The Echo Nest API
Donovan Preston's CoreAudio Python API