SystemDynamics is a Modelica library for the simulation of system dynamics diagrams. It is based on a version from the textbook "Praxis der Simulationstechnik", but has some minor behavioural changes in the timing of discrete events compared to the book version. It is provided as open source under the GNU GPL license (version 3).
If you are interested in library internals, have a look at the following paper. An extended english version is in preparation.