Homeschool Math
Homeschool Math: Parallel and Perpendicular Lines
This lesson explains what are parallel and perpendicular lines and includes varied exercises and a video that shows how to draw a perpendicular line and a rectangle using a protractor or a triangular ruler.
University of Georgia
University of Georgia: Inter Math: Perpendicular Lines
This site from InterMath opens with a very simple definition of perpendicular lines and has links to challenging problems, more information, related terms and everyday examples of perpendicular lines.