Wolfram Research
Wolfram Mathworld: Definite Integral
Good site explaining the definite integral. This site also gives the integration rules for Definite Integrals, along with definitions and examples.
Interactive Mathematics
Interactive Mathematics: The Definite Integral
This demonstration allows you to see the process of finding the definite integral of a function, then put that process to practice in several examples. The definite integral is applied to real world situations such as work and average...