Maths taught the way it should have been the first time: intuition and analogy first, formulas afterwards. The essays on e and the Fourier transform are the classics.
Plain-language, illustrated explanations of maths from counting to calculus, with puzzles and little interactive tools. Still the clearest on the web.
Probability and statistics taught as things you drag: sample from a distribution, watch a regression line settle, see the central limit theorem happen in front of you.
Simple maths, reading and art games for young children, with virtual manipulatives that teachers actually use in class.
