Cumulus is a software synthesizer which implements the Asynchronous Grain Synthesis. This synthesis allows you to create very original sounds to be used in your music (mainly percussive and ambient background sounds). It features a convenient QT-based GUI where each paramter can be controlled by an envelope, the ability for all parameters to be controlled with hardware controllers (MIDI-controllerbox), an interface to use a MIDI-keyboard (polyphony adjustable, default 8 voices), internal 32-bit accuracy, an internal sampling rate that can be any multiple of the external one, and full realtime capability with parameter changes audible within around 50ms, depending on system latency.
Cheese Tracker is a program to create module music. It aims to have an interface and feature set similar to that of Impulse Tracker. It works on Unix systems and on Windows (using cygwin), but all the interface/audio code is fully modular and abstracted in individual classes, which should make the porting of this program to other platforms very easy.
SpiralSynth Modular (or SSM) is an object orientated modular softsynth / sequencer / sampler. Audio or control data can be freely passed between the plugins, and is all treated the same. Data can also be fed back on itself for chaotic effects. LADSPA plugins are supported, and you can use them in the same way as any of the native plugins. Not having MIDI won't restrict what you can do. It currently uses the standard OSS/Free sound input/output (/dev/dsp), and should work on ALSA too. You can also render WAV files directly to disk.
jMax is a visual programming environment for building interactive real-time musical and multimedia applications by allowing the user to interactively design dataflow circuits. The basic data types that can go through are integers, symbols, lists, etc. It is an event-driven system and has been used for MIDI processing. A second part of the system (DSP) allows a continuous signal to flow through a circuit, which is most useful for PCM sound (ie. microphone, sound files, etc.). The system is extensible by using shared libraries, you may add data processor types, data types, GUI elements, device types, and more. Data processors may also be designed as circuits and reused.
Trommler is an X-based drum machine with the following features: use of 16 bit 44100kHz mono drum samples, realtime audio output using the /dev/dsp device on Linux machines (or audio output to file), a variable number of beats per pattern, volume adjustment of drum samples, stereo panning of (mono) drum samples, and more.
Swami is an instrument patch file editor using SoundFont files that allows you to create and distribute instruments from audio samples used for composing music. It uses FluidSynth, a software synthesizer, which has real time effect control, support for modulators, and routable audio via JACK. This project supersedes the Smurf SoundFont Editor, and is an entire object-oriented rewrite of it. The supporting libraries are GUI-independant and can be used in your own programs for doing SoundFont manipulation.