Wolfram Research
Wolfram Math World: Calculus and Analysis
Over two dozen links are provided at this MathWorld site. From Polynomials and Series to Dynamical Systems and Catastrophe Theory, each link provides in-depth explanations and relevant diagrams of the topics at hand.
Science U
Science U: Isometries: Reflection
This site from Science U gives a good example of reflection of symmetry. It also gives some background information about reflection as well.