Formula and description of the perimeter of a parallelogram.
Math Open Reference

A helpful scientific calculator that runs in your web browser window.
Holt Online Learning

Parallel Lines, and Pairs of Angles
math is fun

This site outlines the theorems about parallelograms with proofs to these theorems.
Steve Wilson of Sonoma State University

A page with links for properties of circles and different types of polygons. Each link has great visuals with clear markings, definitions, and formulas.
Cool Math

This interactive quiz allows you to test your knowledge on complementary and supplementary angles, as well as all the special angle pairs with the transversal of parallel lines.
Mr. Martini's Classroom

An interactive applet showing the way the angle values change but the relationship between angle pairs does not.
Saltire Software Geometry Gallery

This game (similar to "Who Wants to be a Millionaire") reviews parallel and perpendicular lines and their slopes.
Quia

This "Jeopardylike" game reviews basic geometry vocabulary and parallel lines/angles vocabulary
Quia
