ID3.py is an object-oriented Python module for manipulating the so-called ID3 informational tags on MP3 files, which include such data as artist, track title, genre, et cetera. ID3.py is extremely easy to use, and is intended for folks who are developing MP3 encoder wrapper programs or browsers in Python.
IRMP3 is a multimedia audio jukebox for Linux. IRMP3's strength is that it easily integrates into home, car, and mobile environments (where a keyboard/screen interface may not always be available). It supports several input and display devices such as infrared remote controls, LCD displays, network connections, and serial keypads. It includes a robust set of tools like alarms, sound environments, mixers, sleep functions, and more. It supports MP3 players and network-ready control & status monitoring, and includes a flexible, modularized source which allows functionality to be easily extended with user-developed code.
jac is a command line CD player that allows you to play your CDs in any order with looping features (on track, playlist, or the entire CD), random playing, backwards playing, exclude from playing, crazy-playing, and many other features. It supports the Italian and English languages, CDDB, and CDIndex.
Jack is a console based cd-ripper. It supports cdparanoia and cdda2wav (amongst others) for extraction and various MP3 and OGG/Vorbis encoders; you can even rip from a cdrdao-generated CD image. Emphasis is on failure-safeness and ease of use. Jack can encode multiple tracks at once, good on SMP systems. It can resume its work if interrupted and keeps an eye on available HD space. Freedb query is also supported, any time you like, even after encoding. Jack can try to query freedb for MP3s of which you do not know the CD anymore, provided you have a complete rip and know the track sequence.
JNapster is a Napster client implemented in Java. It allows you to download MP3s over the Internet. It provides a GUI similar to that of the Napster Windows client and has some handy features the original doesn't have: It gets server lists directly from napigator.com and logs you into as many Napster servers as you want, which you can query in parallel.