Physyn is an n dimensional physical modelling synthesizer written in C.
What you do is define a model (say a string or a drum skin) and an action (say plucking or hitting it) and physyn will work out what sound would be produced by doing that action. Since it does the simulation in real time (for now) successive actions interact with the model realistically.
In terms of implementation it uses a points and springs based modelling method.
It depends on Jack audio.
When I get around to documenting it I will link it here.
Plans for the future include midi input (next on the todo list), a blender plugin for model creation, decoupled rendering (over/under sampling and offline rendering) and a higher level model description language.
The code is on Github.