Java Applet.
I did not make this applet, Andrew Duffy is the creator! i only remixed it. It is exactly what we need in the Singapore syllabus as well. nice!
mirror here as well

changes made: was at the airport after 2010 AAPT Summer Meeting in Portland, Oregon, at PDX airport from 11 pm 21July to 6 am 22July, made used of the time to customized this applet.
1 redesign the look and feel to be all buttons and slider on the bottom panel
2 color scheme for red is + charges, blue (64,64,250) for - charges even visible for the slider now
3 resized to 600,600 for
4 added forces F1 an F2 when show2&&testc==false
5 added symbolic representation of Enet and Vnet
6 added KE = 0.5*m*v^2 ;  PE = q*Vnet;  TE = KE+PE; due to question by leeyiren
03 August 2011
made some changes add input field to support the sliders
rearrange the check box
add a step () button
add color codes for the test charge as well, red = +, blue = - grey = 0
add range of view if test change move outside current xmin and xmax using the codes
[code]if ( xtest < xmin ) xmin = xmin*1.2;
if ( xtest > xmax ) xmax = xmax*1.2;[/code]
make the graphs to plot to go to xmin and xmax instead of the older -6 and 6 range