I'm always a little depressed when someone beats me to writing a cool article, and this one was on my list. In this case, however, Adam Pash at Lifehacker has done for many consumer-grade Canon point-and-shoot digital cameras. , for Canon Hacker's Development Kit, is a non-destructive firmware enhancement that adds six categories of features:
CHDK works with a number of Canon models, though not all of them, so you'll need to check before going any further (and no, as far I can tell, no other manufacturer's cameras have any CHDK-like hacks). What's especially nice about CHDK, apart from all the useful functionality it provides, is that it modifies the camera's firmware only when you explicitly load it, and everything is back to normal when you next power up the camera. Have fun hacking!