Differential equations
Applets
The first-order differential equations: Plots the slope fields for any first order differential equation. [IES]
Direction field applet: A much better slope field plotter -- it has more bells and whistles. [Dartmouth]
Interactive differential equations: Check out the first three topics: Newton's Law of Cooling, Graphic Differential Equations, and Single Species Populations. [Addison Wesley Longman]
Separable differential equations: Nice explanations of basic problems, and some nice applets, too. [University of British Columbia]