Wolfram Research
Wolfram Math World: Orthocenter
This site from MathWorld provides a diagram of a triangle with its orthocenter labeled. The site details how to find the orthocenter with links to the key terms. The site details many other interesting properties of the triangle...
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Characteristics of an Orthocenter
Here at The Math Forum, Dr. Math offers interesting properties of orthocenters. Links are provided at the bottom of the page for information on associated topics.