Other
University of Evansville: Encyclopedia Triangle Centers
This site contains a comprehensive study of triangle centers. Seems more for an advanced level. Provides a list of six links to 1114 centers.
National Council of Teachers of Mathematics
The Math Forum: Ask Dr. Math: Triangle Centers: Etymologies
At this page you will find Dr. Math responding to an inquiry about the relevance of the names of each of the special triangle centers.
Math Words
Math words.com: Altitude of a Triangle
This site provides a definition of what the altitude of a triangle is. It also provides a diagram.