Lyra is a front-end to the Music Player Daemon (MPD). Based loosely on the Creative Media Source software, it has the aim of supporting large collections of albums and artists. Other interesting features are the implementation of a local cache of the music library using an SQLite database, increased speed, and flexibility. In addition, Lyra supports integration with the Last.fm service, which includes "Scrobbling" and album cover art.
Sfront compiles MPEG 4 Structured Audio (MP4-SA) programs into efficient C programs that generate audio when executed. MP4-SA is a standard for normative algorithmic sound, which combines an audio signal processing language (SAOL) with score languages (MIDI and SASL). Under Mac OS X and Linux, sfront supports real-time, low-latency audio input/output and MIDI input. Under Mac OS X, sfront supports the creation of AudioUnit plug-ins. The documentation includes a book about SAOL programming.
iTunes Export provides the ability to export iTunes Playlists to the standard .m3u file format. Unfortunately, this feature is not included in the iTunes application and is extremely useful for users who wish to use iTunes with other applications or devices. iTunes Export is written in C# (.Net). iTunes Export provides a Command Line version (tested on Mono) and a GUI version, as well as a library that developers can use in their own applications.
Herrie is a command line music player. It has a split-screen file manager and playlist interface and supports a number of file formats (MP3, Ogg Vorbis, wave, FLAC, etc). It also has some nice features, including a chroot() function for untrusted setups (remote logins, parties, etc). AudioScrobbler is also implemented using Curl.
Podtool is a Python-based utility for managing tracks between a local music repository and an iPod. In particular, it implements correct playcount and ratings synchronisation, which is important when using smart playlists. If you've used iTunes and selective smart playlist synchronisation, then you know what this does.