Twibright Syntring is a purely algorithmic string synthesizer with ASCII tab input and wav output. It doesn't use any samples, yet sounds quite realistic. Timbre, volume, and timing are adjustable in the tab file. It can be used for learning or practicing guitar or bass playing, for editing and synthesizing one's own tunes, or producing MIDI sound fonts. It is very lightweight and portable, and doesn't require any libraries.
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.
bcr harms takes a model of the quantum harmonic oscillator and adapts it as a soft synth. It lets you hear and see how the oscillator wave functions evolve in real-time as you modify amplitudes and other parameters. The user interface is based on and integrates with the BCR2000 MIDI controller, but the application can be used without a controller.
bcr beats is a beat slicer that lets you cut up and play with an audio sample: you can trigger slices and loops with MIDI or the mouse and keyboard, set pan, volume, and sample rate values, and generally have lots of fun. The interface is based on the Behringer BCR2000 MIDI controller, which means lots of knobs. It can be used with or without a controller.