lifehacker.com have posted an excellent article on the Canon Hacker's Development Kit. CHDK is a free, open source project with which you can add features like RAW shooting mode, live RGB histograms, additional photographic settings, motion-detection, time-lapse, longer video recording and even games onto your existing Canon camera. The beauty of CHDK is that it is non-permanent and non-destructive, you can use when you want to; CHDK makes no changes to your camera. Installing CHDK entails having a couple of files added to your camera's memory card. Removing it involves simply restarting your camera without the memory card present.