Major CHDK features

* RAW – CHDK can record raw files, giving you access to every bit of data the sensor saw, without compression or processing. Raw files can be manipulated on the camera, or processed on your PC. CHDK also has experimental support for the open DNG raw standard.
* Override Camera parameters – Exposures from 64s to 1/60,000s with flash sync. Full manual or priority control over exposure, aperture, ISO and focus.
* Scripting – Control CHDK and camera features using ubasic and Lua scripts. Enables time lapse, motion detection, advanced bracketing, and much more. Many user-written scripts are available on the forum and wiki.
* Motion detection – Trigger exposure in response to motion, fast enough to catch lightning.
* Edge overlay – Detect the edges in a scene, and display them later. Ideal for timelapses, stop-motion, stereography and much more.
* Live Histogram – CHDK includes a customizable, live histogram display, like those typically found on more expensive cameras.
* Customizable OSD – Improved display of battery status, free space, camera parameters, and much more. Fully customizable with an on-screen editor.
* Filebrowser – Manage files without a PC.
* Games – Play Reversi, Sokoban, Mastermind or 4-in-a-Row on your camera.
* USB remote – Simple DIY remote allows you to control your camera remotely.
* User Menu – Edit your own customizable User-Menu for fast access to often used features.

Up for it? There is a guide on Lifehacker, and you can go to the CHDK site for more info as well.