photocritic tweeted earlier about a couple of Camera Simulators - i.e. simple tools which allow you to alter the aperture and shutter speed on a fixed image and show what the various affects will be.

My favourite of the two mentioned is this one, which says about itself:

With this camera simulator you can try different aperture and shutters to affect photography’s light, depth and motion blur. Try also to change the camera’s light sensitivity by manipulating its ISO value. A higher ISO value gives higher sensitivity to light, which allows faster shutter speeds. The disadvantage is that the picture is more grainy.

Seems like a good way to understand the relationship between these three key elements to photography