SoftSynth.com

Music and synthesis software API, including JSyn and JavaSonics ListenUp.
http://www.softsynth.com/

Zerius Development Inc.

Java program for designing and generating sounds on-line. Also a vocoder for download.
http://zerius.com/

JavaZoom

Java MP3 Player supports WAV, AU, AIFF, Ogg Vorbis also.
http://www.javazoom.net/

JavaTuner

Tunings and temperaments. Demo applet.
http://pages.globetrotter.net/roule/accord.htm

jOggPlayer

Gui Enhanced Vorbis Ogg Player. Freeware.
http://jOggPlayer.webarts.bc.ca/

FreeTTS Speech Synthesizer

Written in Java, based upon Flite 1.1: a run-time speech synthesis engine from Carnegie Mellon. Open source, BSD.
http://freetts.sourceforge.net/docs/index.php

Xenoage MusicXML Player

Applet and standalone application that plays MusicXML files and converts them into MIDI format.
http://mxmlplay.xenoage.de

MuSing

Aids the creation of musical rhythms.
http://www.geneffects.com/musing/index.html

Neuralsemantics Modulator Jane

Applet uses standard AU sound files to modulate graphics on web page. Also offers Neurosonic e-Guitar.
http://neuralsemantics.com/mjane/mjprod.html

Sonicle Audio

WebSynth product line. Applets, players. Some freeware.
http://www.sonicle.com/www/index.jsp?id=F5554B38BEF7C9A8D3D498E77DA8A446&language=en&pagename=websynth

JFugue

API for specifying music using simple strings and easily transforming patterns of music.
http://www.jfugue.org

RPitch

Relative pitch ear training to recognize intervals. Freeware.
http://rpitch.sourceforge.net

FreeMusicPlayer

Part Java part native, supports MP3, WAV, Ogg, and playlists. Windows 2000/XP.
http://patriot.net/~gary/FreeMusicPlayer.html

Frinika

Cross-platform music studio software written in Java. Includes sequencer, software synths and realtime effects.
http://www.frinika.com