RTflow is a free, lightweight dataflow modelling tool for real-time systems, somewhat similar to tools like MathWorks Simulink, National Instruments LabVIEW and SystemBuild. In RTflow, you build your system in a graphical language that is both parallel and deterministic, so that threading and timing problems are completely avoided. You verify your model in a simulator that produces graphs of every variable in the system, which allows you to spot bugs and their causes effortlessly. Finally, you generate software (C, C++ or Java) or hardware (VHDL) code from the model, allowing for immediate implementation on your target platform.
Read more about RTflow in the following articles, or go straight to the download page to try it out.