IMMS (Intelligent Multimedia Management System) is an intelligent playlist framework that tracks your listening patterns and dynamically adapts to your taste. It is incredibly unobtrusive and easy to use as it requires no direct user interaction. Plugins are currently available for XMMS and BMP.
songanalysis analyzes a song in WAV, Ogg Vorbis, or MP3 format and produces an output consisting of the volume differential, the relative strength in each frequency band, and the tempo. This is useful if you want to build something that lets you find out whether you have duplicate files, if you need to find out the tempo of your songs, or if you need to find similar songs according to how they sound.
Recording level monitor records analog audio from the sources you choose and plays it back simultaneously so that you can hear it, while monitoring the loudness (input) level of the recording. You'll be able to hear and see whether the volume is too high (introducing analog distortion and digital clipping) or too low (letting through background noise and hiss) on your audio gear.
SlimScrobbler is an slimserver plugin that builds a profile of your musical tastes by sending the name of every song you play to the Audioscrobbler server, which updates your musical profile with the new song. Every person with a plugin has their own page on the site that shows their listening statistics, and the system automatically matches you to people with similar musical tastes, and generates personalized recommendations.