Surfaces and partial derivatives
Animations
Interactive animation: Several basic surfaces, from paraboloids to hyperboloids to ellipsoids to cones. [Paul Blanchard]
Interactive animation: Plot of a discontinuous function of two variables. [Paul Blanchard]
Applets
Plotting a function of two variables: Plots the graph of any fuction of 2 variables, allowing for real-time rotation. [Vanderbilt Toolbox]
One-variable derivative plotter: Plots any curve, along with a moving tangent and graphs of its first and second derivatives. [MIT]
Tutorials
Partial derivative counterexamples: Plots and explanations of "pathological" surfaces from Calculus III. [Tom Vogel]