Battle of the Hardware-Boosting Hacks is Turn Your Point-and-Shoot into a Super-Camera:
"If you're using a consumer grade point-and-shoot Canon digital camera, you've got hardware in hand that can support advanced features way beyond what shipped in the box. With the help of a free, open source project called CHDK, you can get features like RAW shooting mode, live RGB histograms, [longer exposure times and faster shutter speeds,] motion-detection, time-lapse, and even games on your existing camera.
...CHDK can run small scripts that will allow your camera to perform a set of actions based on the conditions of the script. Tons of scripts are available, and they provide functionality like motion-sensing photography (which reportedly works for lightning strikes) and unlimited interval time-lapse photography."
Update: Prolost (6-22-09) notes the Magic Lantern firmware hack for the Canon 5D Mark II that enables: Onscreen audio meters, Zebra stripes, Crop marks, Manual audio level controls, Lower noise than stock firmware, etc.