Some links I like.

The Program of Mathematics Education

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

Math Forum(alias Geometry Forum)

Sine Functions |
Quadratic Functions |
Graphing Quadratics in the xb plane |

Centers of a Triangle |
Constructions |
Construction of a Parabola |

Tangent Circles |
Altitudes and Orthocenters |
Pedal Triangles |

Parametric Equations |
Polar Equations |
Mortgages |