iCommune is an application that extends Apple's iTunes to share music over the network. It allows you to share the music in your iTunes library and access other iCommune music collections. iCommune music collections appear as playlists in your iTunes window. You can browse through them, and choose to stream or download the music they contain.
Audiality is a highly scalable and portable audio engine and synthesizer. MIDI files are used in combination with scripting and modular synthesis, to minimize file sizes and maximize flexibility. Audiality can be used as a music and sound effects player in multimedia productions, or as a realtime MIDI synthesizer.
MidiKeys is a little application that displays a piano keyboard onscreen. You can click the keyboard or use the computer keyboard to play MIDI notes. The keyboard window can be made to float on top of other applications and supports transparency. Also, MidiKeys will optionally set up hotkeys for the computer keyboard so you can play notes in other applications (such as a sequencer) without having to switch back to MidiKeys. It works great on a PowerBook!
SynthTest is a somewhat simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit, and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used. SynthTest has a reference implementation of preset support for Audio Units. It is primarily intended to be used by Audio Unit developers for testing purposes (the source is available), although it can be great fun to play with all by itself. It is complete enough to be used for live performances.
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.