clipped from: howto.wired.com 
CANON CAMERAS ONLY !!!!!
CHDK, the Canon Hacker's Development Kit, is an open-source software project that can be loaded on cameras using Canon's DIGIC II or DIGIC III firmware platforms. It unleashes new features including RAW file format, live histogram display, a battery readout, and the ability to run scripted actions on a camera.
CHDK does not replace the existing firmware on your Canon, so the process is completely reversible. The existing firmware stays intact, while the CHDK software is loaded on demand from an SD card.