Harvey Mudd College
Harvey Mudd College: First Derivative Maxima and Minima
Shows how to find absolute extrema using the first derivative test.
Mathematics Archives
Math Archives: Visual Calculus: Drill First Derivative Test
A good worksheet for students to practice their first derivative skills. Allows you to check your answers by clicking the "solution" box.
Wolfram Research
Wolfram Math World: First Derivative Test
This MathWorld site shows how to find extrema using the first derivative test.