This is just awesome.  It lets you see the effects of everything we talk about in our Exposure workshop in an online simulation!

http://camerasim.com/camera-simulator.html

You can change your aperture, shutter speed, focal length and more. You have a simulated viewfinder including all of the information you would normally see, such as your exposure indicator, current settings, under/over exposure warnings, etc.

Change all your settings, hit the shutter, and see the results!