ExplodingDots.org is mind-blowing! The site description: "Exploding Dots is the name of an astounding mathematical story that starts at the very beginning of mathematics — it assumes nothing — and swiftly takes you [on] a wondrous journey through grade school arithmetic, high-school polynomials algebra, infinite sums, and advanced mathematics and unsolved research problems baffling mathematicians still to this day."
Short instructional videos by Dr. James Tanton, math ambassador extraordinaire, and accompanying text explain the ideas. Interactive exercises let you try out the techniques. It's amazing how something so simple can help visualize math from elementary grade concepts through college math. Teacher materials are included. Good for all ages, and highly recommended!