CHDK stands for "Canon Hacker's Development Kit" and is the name of a firmware enhancement to the consumer level point-and-shoot Canon "PowerShot" cameras. Some very bright people got together and figured out a way to unlock some features that these little cameras are capable of but don't come standard out of the box.
The more knowledgeable photographer may appreciate the ability to take RAW photos, the longer exposure times and faster shutter speeds, among other things. The hobby photographer may be fascinated by the ability to take time-lapse photos in the garden, longer video recording and the camera's ability to take photos when it detects motion.
Recently, I've seen that a digital camera marketed to birders has been getting a lot of press and recommendation. At over $200.00 the price tag seems a little steep to me considering that the Canon PowerShot camera can be hacked to do basically the same thing.
Photographers have been doing some very cool things with the CHDK- like attaching cameras to bikes, cars and kites. A CHDK search of Flickr show many groups and photos devoted to CHDK photography. A similar search of YouTube also yields some amazing videos.
If a Canon PowerShot was your first digital camera and you've graduated onto something bigger and better fish it out of your junk drawer or take it away from the kids and put it to new use in the garden.
Time-lapse video of cress seeds sprouting.