Electric Fields Applets

In 2-d:

http://falstad.com/vector2de/

In 3-d (much cooler):

http://falstad.com/vector3de/

Also worth a look:

http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

http://www.cbu.edu/~jvarrian/applets/efield1/elefi_z.htm