Wolfram Research
Wolfram Math World: Vanishing Point
MathWorld offers a definition of vanishing point; contains diagrams of one, two, and three point perspective drawings with arrows pointing to the vanishing points.
Other
Hop's Gallery: Perspective Study
This webpage explores traditional perspective drawings as well as contemporary drawings, focusing on their use of horizontal and vertical lines.
Other
Harold Olejarz: Interactive Horizon Line
An interesting interactive slide show about perspective, including the horizon line and vanishing points.