Lots of fun stuff!
Sing along! The Square of the Hypotenuse
This website lets you calculate Riemann Sums: Riemann Sum App
This and this one finds solids of revolution: Volume App
And another solid of revolution animation: Spin It Again!
A great article on the history of the integral: It Slices, It Dices!
And what of series expansions? Here's an animation of a very useful expansion,
the Fourier transform: Fourier Series
Circular Motion and Trig Functions: Animation
And a more general article on expansions... the "Swiss Army Knife of Mathematics!"
Make slope fields instantly: Slope Field Generator
Need help with statistics? Here's a place that can help: Stat Trek
Do calculus without all that math! Use The Mechanical Integrator