Supercharge Your Canon with Open-Source Firmware
I think the most important feature for stock photographers is that it enables RAW shooting on the camera, but also live histogram view, battery readout, and the ability to run scripted actions on the camera and going with the shutter speed beyond the limit allowed by the original firmware.
Full article on where to get it and how to install it here
I haven't tried it yet, but I will on my wife's S3 IS.
Photo credits: Patrick Hall.