SageModeler is a free interactive graphic design tool students can use to diagram the structure of a system, runs simulations and much more.
Use the drag-and-drop features to also create static equilibrium or time-based dynamic models. Students can add data to run simulations without having to write code or mathematical formulas.
This tool is being developed by the Concord Consortium at Michigan State University, specifically for middle and high school students. It includes curriculum units and a teachers guide.