I wrote a fun program a few years ago that can draw electric field lines. You can place positive and negative charges on the screen, set the strength of the charge, and draw the field lines. It can also draw the voltage contour lines, and export to Excel for 3D visualization.
There are still more features I'd like to add, but this program really has potential (groan).
As of 2016, this program still works well in modern versions of Windows.