This is an open-source addition to the firmware of point and shoot Canon cameras based on the DIGIC II and DIGIC III firmware platforms. It isn't a full replacement, nor is it permanent so no harm can be done to the camera. The firmware extension is loaded on the fly from the SD card.
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.