those that have been around for a while will probably remember this one... the difference now is kyou get to play with a demo and the full version has to be purchased... bugger
http://www.scopesim.com/
it lets you play with telescope aperture sizes, FL, and eyepieces to give you an idea what sort of FOV you should be seeing under different limits
its fun to fiddle with, unfortunately you olny get a 1/2 moon to play with.