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Wolfram Alpha is like a search engine driven by math. Plug in problems or questions and see what it gives you. Try inputting your birthday...
Desmos Graphing Calculator is a great free online calculator.
Khan Academy gives some good video lessons on a variety of subjects.
*Tambien obtener videos de matematicas utiles en español! Khan Academy en Español
Math Open Reference offers some great interactive tools for exploring math.
MathWorld is the best web math encyclopedia. It can be a bit tricky to read, though.
MathNEXUS offers challenging puzzles and problems, some history, some humor, and lots of other ways to explore math.