Vector Field Graphing Utility

This is a great tool for plotting vector fields:

http://dlippman.imathas.com/g1/fullgrapher2d.html

You can also plot functions of all kinds:  implicit, explicit, polar, parametric, etc.  In addition, you can plot several different graphs simultaneously (say, a vector field and a set of flow lines), and you can modify each graph individually.

I found this very helpful is constructing a recent test!