id3j is a lightweight and easy-to-use library to create and add ID3 metadata tags to MP3 files. It only concerns itself with ID3 version 3.2.0 and only handles the most common types of information, but creating a tag and adding it to a file is much easier than with many other solutions. It also has one fairly esoteric feature that is interesting from a algorithmical standpoint, which is the ability to create CRC32 neutral ID3 tags. That means that you can add such a tag to any MP3 file and the CRC32 sum of the resulting MP3 file with the tag will be the same as the file without the tag.
qTunes is a modular music player designed to be simple to install, configure, and use, with a small footprint for low-power systems. qTunes features an AJAX Web interface using Extjs, which can easily handle over 40000 tracks with searching and sorting. It supports multiple zones, allowing you to play different songs in different areas simultaneously. It can stream to Apple Airport devices, including synchronizing over multiple Airports. It uses the DAAP protocol to share music with iTunes. It uses the DACP protocol to allow the player to be remotely controlled by the Apple Remote.app available on iPhone and iPad. qTunes currently plays MP3 and MP4 only, but its modular design allows easy plugin of support for AAC and other formats.
Webplay creates, from a collection of MP3s and Ogg Vorbis files (indexed from a directory or MySQL/PostgreSQL database), a Web-based jukebox with support for multiple independent streams. If Lame/oggenc are available, it can change the bitrate or codec of a file in real time to a selectable value. Each user controls their stream via a Web interface and can skip back, forward, or to a specific track; increment or decrement the bitrate; and check the current playlist, song, or bitrate. Playlists can be looped, randomized, played locally on the webserver, edited, deleted, reused, or renamed.