Wolfram Research
Wolfram Math World: Triangles
This site provides detailed overviews of many of the concepts and theorems associated with triangles. Content focuses on general triangles, cevians, triangle centers, triangle circles, and triangle lines.
National Council of Teachers of Mathematics
The Math Forum: Triangle Formulas
Investigate the geometric definitions of triangles, focusing upon equilateral, isosceles, right, and scalene with detailed drawings and appropriate formulas for each. This site has links to more information on triangles.