Some links I like.
The Department of Mathematics Education
MacTutor History of Mathematics Site  St. Andrew's University
Math Forum (alias Geometry Forum)
y = asin(bx+c)

Exploring

Further exploration of

Centers of a Triangle

GSP Script Tools

Three points & equal segments

Tangent Circles

Altitudes & Orthocenters

Pedal Triangles

Parametric Curves

Polar Equations

EXCEL Spreadsheet & Graph 