annoyme is a program that plays a sound effect every time you hit a key on your keyboard to give you the feeling you are using a real typewriter. This can be used to impress colleagues with your typing skills or just for sentimental reasons. It supports various sound input methods, as well as multiple output libraries like ALSA or AO. Most importantly, it is also possible to add more sound themes. Currently, only input for XEvIE (on an X Server) is supported.
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.