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).
Download from here, no installation needed, just unzip and run electrocity.exe.
Source code is here, released under the GPLv3.
As of 2016, this program still works well in modern versions of Windows.