Plane Geometry

Occurrence of the Conics
Images of conics by Jill Britton.
http://britton.disted.camosun.bc.ca/jbconics.htm

Polygons
Discusses the differences between the different types of these straight sided figures. Also, explains the difference between regular and irregular shapes.
http://www.ul.ie/~cahird/polyhedronmode/polygons.htm

Pythagorean Theorem
A collection of 43 proofs - some interactive - of the Pythagorean theorem.
http://www.cut-the-knot.org/pythagoras/index.shtml

Tim's Triangular Page
Explains the basic properties, formulas, theorems and different types of triangles. Includes proofs and illustrations.
http://sakharov.net/triangle.html

Xah: Visual Dictionary of Special Plane Curves
A Visual Dictionary of Special Plane Curves with graphics.
http://www.xahlee.org/SpecialPlaneCurves_dir/specialPlaneCurves.html