The integral
Applet
Riemann sums applet: Approximate the area under the curve of any function with rectangles using the various left- and right-hand style rules. [David Eck]
Riemann sums: A tutorial on calculating Riemann sums using your TI-85 or TI-86. [Visual calculus]
Computing Riemann sums: Computes Riemann Sums for you. [Vanderbilt toolbox]
Area under a curve: A tutorial which includes a couple of applets illustrating Riemann sums and areas under curves. [University of British Columbia]
Definite integral of a function: Computes definite integrals for you. [Vanderbilt toolbox]
Online problems
Problems: Approximating areas under curves. Includes graphs and solutions. [Visual calculus]