Mathlets
There are a number of useful mathlets, or math-based applets, out there on the web. These can be used in class for demonstrations, or as labs to get students to explore ideas. Here's my collection of them.
Differential calculus
General help \\ The zoo of functions \\ Limits of functions \\ Continuity \\ The derivative \\ Derivative toolbox \\ The chain rule \\ Higher-order derivatives \\ Implicit and logarithmic differentiation \\ Linear approximation \\ Related rates \\ The shape of the graph \\ Curve sketching \\ Optimization
Integral calculus
The integral \\ The Fundamental Theorem of Calculus \\ The method of substitution \\ Areas and volumes \\ Differential equations \\ Partial fractions \\ Trigonometric substitution \\ Arc length and surface area \\ Work integrals \\ Improper integrals \\ Parametric equations \\ Polar equations \\ Sequences and series
Multivariable calculus
Plotting tools \\ Parametric equations \\ Polar equations \\ Vectors and vector equations \\ Lines and planes \\ Surfaces and partial derivatives \\ The gradient and its uses \\ Extrema: local and constrained \\ Double and triple integrals \\ Integrals over nonrectangular domains \\ Line integrals