Nyquist is an elegant and powerful language for sound synthesis and music composition. Unlike score languages that tend to deal only with events, or signal processing languages that tend to deal only with signals and synthesis, it handles both in a single integrated system. It is also flexible and easy to use because it is based on an interactive Lisp interpreter. You can design instruments by combining functions (much as you would using the orchestra languages of Music V, cmusic, or Csound). You can call upon these instruments and generate a sound just by typing a simple expression. You can combine simple expressions into complex ones to create a whole composition. It runs under any Unix environment, MacOS, Windows 95, and Windows NT, and it produces sound files as output (or direct audio output under Windows).
The CMU Midi Toolkit (CMT) is a collection of software for writing interactive MIDI software in C. It includes a number of handy utilities along with an application "shell" that provides timing, scheduling, and MIDI interfaces that are portable across DOS, Mac, SGI, and Amiga platforms.
Signal Applications To Audio Networks is a program aimed at different signal processing tasks. The primary type of signals are audio signals. The user describes networks of different modules using a special language, and the modules are dynamically loaded libraries. The system can be used to create music from scratch, or to process already created audio files, and depending on the complexity of the network versus the capacity of the host machine, everything can be done in "real time".
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.
The Audio* Music Synthesis Project is music creation software. Currently it features Mojo Synth which is a polyphonic analog modelling synth that has a 64 step sequencer and can also be played from a MIDI keyboard. It attempts to emulate classic analog sounds from Roland M-100, Korg MS-20, and Mini Moog synthesizers.
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.
Javasynth is a Java-based system for creating (rather experimental) modular audio systems called sound modules. This allows synthesizers of some kind to be assembled in a modular fashion using oscillators, amplifiers, envelopes, filters, and other components familiar to those that have worked with hardware synthesizers. It includes a separated synth engine and a Swing user interface for building sound modules.
GtkGEP turns your computer into a realtime effects processor. You can plug your guitar into the computer and play with cool distortion effects, for example. It has a modular plugin structure, with standard plugins including distortion, overdrive, delay, reverb, equalizers, and a flanger. It works in 16-bit resolution, in mono mode, and with frequencies from 11khz to 44khz. The sound quality is very good.