Many physical systems are described by time-dependent differential equations or systems of such equations. This makes it difficult for students in an introductory physics class to solve many real-world problems since these students typically have little or no experience with this kind of mathematics. In my high school physics classes, I address this problem by having my students use a variety of software solutions to model physical systems described by differential equations. These include spreadsheets, applets, software my students themselves create, and systems dynamics software. For the latter, cost is often the main issue in choosing a solution for use in a public school and so I researched no-cost software. I found Sphinx SD, OptiSim, Systems Dynamics, Simile (Trial Edition), and Vensim PLE. In evaluating each of these solutions, I looked for the fewest restrictions in the license for educational use, ease of use by students, power, and versatility. In my opinion, Vensim PLE best fulfills these criteria. (Contains 9 figures and 2 tables.)