Illuminations is a project designed by the NCTM (National Council of Teachers of Mathematics. It includes interactive tools for students and instructional support for teachers.
Desmos is the next generation of the graphing calculator. The calculator can instantly plot any equation, from lines and parabolas up through derivatives and Fourier series. Data tables open up a world of curve-fitting and modeling. Sliders make it a breeze to demonstrate function transformations.
Geogebra lets you explore algebra and geometry, much like it’s proprietary cousin, Geometer’s Sketchpad. Geogebra requires Java to run, and can either run in a web browser with no download, or you can install it to your computer using the offline installer.
Dreambox Learning is an adaptive software learning environment. This link provides (for free) a sample of some of their K-8 interactive tools . Lessons can also be downloaded.