**Some links I like:**

The Department of Mathematics Education

MacTutor History of Mathematics Site -- St. Andrew's University

Math Forum(alias Geometry Forum)

Explorations
of the Sine Curve |
Explorations
of Second Degree Functions |
Explorations
of Quadratic Equations |

Explorations
of Triangle Centers |
Geometer's
Sketchpad Script Tools |
Explorations
with Geometer's Sketch Pad |

Tangent
Circles |
Orthocenters
and Circumcircles |
Pedal
Triangles |

Parametric
Equations |
Polar
Equations |
Explorations
with Spreadsheets |