This java applet displays the electric field in various situations. You may select one of many fields from the Setup menu in the upper right.
The applet shows the potential surface of the field, with particles following the field vectors. You may click and drag with the mouse to rotate the view. Also some of the field selections have parameters which may be adjusted.
There is also a 3-D version of this applet (a version with 3-D fields, that is). This version only does 2-D fields, but unlike the 3-D version it can also display the potential surface and demonstrate Gauss's law.
Zip archive of this applet.
Version 1.3, posted 2/22/05