This applet from MIT allows you to select from 8 examples of differential equations. It allows you to select the step size. As you toggle through, the step size you selected traces out the "solution" in the vector field. You can then change the step size. It demonstrates the value of a small step size and also provides the actual solution.
This site seeks for teachers to enroll in its courses, but this is
a page of interactives that are free to use for educational purposes.
It also usually has "warm ups" for learning about the applet and "sample
activities."
A collection of math related topics from basic arithmetic to geometry and combinatorics. Presents the concepts in an easy to understand way, also provides applets demonstrating concepts.