Project-E-and-M is a project mainly written in Python, it's free.
Voltage visualizer
Voltage and Electric field visualizer E and M project by Berit Johnson
This project makes use of Allen Downey's Gui module
Requires Python and Python Visual to be installed
Run em1.py to run
Capabilities:
Able to add or remove arbitrary point or line charges where both line position and line charge density may be defined parametrically
Both point and line charges may be changed and affect the field accordingly
Point charges may be dragged in field and affect the field accordingly
Able to calculate voltage at any given point in terms of k= 1/(4 pi e0)
Able to display voltage field and electric field in three dimensional space
source code may be found at https://github.com/berit/Project-E-and-M