Since 2002, the PhET Interactive Simulations project at the University of Colorado has been working to provide learning tools for students and teachers. The project has developed over 85 interactive simulations--or sims--for teaching and learning science. Although these sims can be used in a variety of ways, they are specifically designed to make scientist-like, inquiry-based activities that are productive and fun learning experiences for students. In this article, the authors focus on the design features of PhET sims that support inquiry learning, highlight their alignment with standards for several science disciplines, and provide examples of inquiry-based activities. (Contains 5 figures and 6 online resources.)