Wired posted a tutorial on installing a custom firmware for Canon cameras utilizing a DIGIC II or III processor that adds or enhances features never before available.
The list of cameras supported are here.Digital cameras have powers beyond what is immediately available to the user. On a standard Canon, for example, the fastest shutter speed option offered is 1/1,600 second, but the hardware can handle much more than that -- up to 1/60,000 of a second.
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.
Full tutorial here.