STK is a set of audio signal processing C++ classes and instruments for music synthesis. You can use these classes to create programs which make cool sounds using a variety of synthesis techniques. This is not a terribly novel concept, except that STK is very portable and completely user-extensible.
Aglaophone is a system of interconnectable modules for the recording, processing, and playback of real-time audio. It features a real-time spectrogram display for visualization along with a number of processing modules. Modules include filters, downsamplers, upsamplers, and quantizers. An MP3 comparison module allows interactive blind comparison of MP3 encoded audio with CD audio, demonstrations of spectral imaging, quantization, and Smith-Barnwell filter bank based wavelet decomposition are included, and there is a module that can perform an automatic spectral analysis of a speaker system.
amber aims to be an easy-to-use granular synthesis tool for Linux to assist composers and electronic musicians in creating interesting and complex sounds. More information on the theory and application of granular synthesis techniques can be found at http://shoko.calarts.edu/~eric/gs.html.
Smurf is a GTK-based SoundFont editor. SoundFont files are a collection of audio samples and other data that describe instruments for the purpose of composing music. SoundFont files do not describe the music itself, but rather the sounds of the instruments. These instruments can be composed of any digitally recordable or generated sound. This format provides a portable and flexible sound synthesis environment that can be supported in hardware or software. Note that Swami is the successor to Smurf and is an entire object-oriented rewrite of it.
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.
The Two Dimensional Spatialization of Sound package takes a single monaural sound signal and processes it to create a binaural signal that places the source of the sound at a selectable arbitrary place around the listener. The package makes use of a set of HRTF Measurements of a KEMAR Dummy-Head Model created by Bill Gardner and Keith Martin of the MIT Media Lab. The sound spatialization software is ANSI C, and can be ported to any platform which will support the sound I/O requirements. The package also includes an FFT written in well-structured C.
Spiral Synth is a physically modelled polyphonic analogue synthesizer. It is capable of creating the kind of sounds made by hardware analogue synths, the noises used in electronic music. You can also use it to make stranger sounds too. MIDI is supported, and it uses the standard OSS/Free sound output (/dev/dsp).