3 links
Pythagorean Theorem
cut-the-knot.org
A collection of 43 proofs - some interactive - of the Pythagorean theorem.
Tim's Triangular Page
sakharov.net
Explains the basic properties, formulas, theorems and different types of triangles. Includes proofs and illustrations.
Xah: Visual Dictionary of Special Plane Curves
xahlee.org
A Visual Dictionary of Special Plane Curves with graphics.