University of Maryland
Evaluating Double Integrals
Comprehensive resource that explains the Double Integral.
Wolfram Research
Wolfram Mathworld: Riemann Integral
This site explains the Riemann Integral and its relation to the Riemann Sum. This site addresses double and triple Riemann Integral scenarios. Includes formulas, definitions, and links to related sites.
Other
Dans Math: Lessons: Calculus (Part 2)
College professor Dan Bach offers a review of the calculus concepts of integrals and vectors. Examples and animated graphs are provided.