Wolfram Research
Wolfram Math World: Second Derivative Test
MathWorld.com has definitions, formulas, and links to relevant sites for derivatives.
Wolfram Research
Wolfram Math World: Extremum Test
With this site one may understand how to test a point in a function for a relative minimum, maximum, saddle point, or none of these, by taking derivatives of the function. Explains and performs what is known as the "Extremum Test."
Wolfram Research
Wolfram Math World: Saddle Point
This site from MathWorld.com gives an example of a saddle point in one dimension along with a definition with links to relative terms and topics.