The best things in life are free, and it is resources like PhET simulations that got me doing sessions on the great freeware and open source resources back in 2005 at ISTE (formerly NECC). PhET simulations, University of Boulder Colorado, is a tried and true FREE resource for Interactive Simulations on Math and Science for grades K-12. And they just announced there is a PhET App for Clever!
Try out PhET, and search by grade level or device then find the topic you are looking for. Not only do they offer an amazing comprehensive library of interactive simulations where students can explore, discover, and embrace math and science concepts, they are very dedicated to accessibility. “Our accessible simulations include: verbal descriptions and feedback, the use of sound and music to represent foundational science and mathematics relationships, and alternative navigation that moves beyond mouse or touch inputs. We are creating research-based, accessible STEM education resources to ensure that all students can experience the benefits of PhET Interactive Simulations.”
PhET Simulations should be in every teachers toolbox.