CHDK stands for Canon Hack Development Kit. This is a free (released under GPL) firmware add-on to the Canon PowerShot firmware that allows you to modify your camera in unimaginabl ^[1]y vast ways. For starters, this includes overriding shutter open-time (from 64s to 1/100K s, with flash sync), R ^[2] AW file capture, live histogram the kind found in really expensive cameras with RGB or ɣ overlay, customizable grids, user-written script support… and much much more ^[3].

In short, as the wiki homepage says, there are quite a few things that could not have been possible without CHDK: lightning photography ^[4], HDR (High Dynamic Range) photography ^[5] and even unlimited DOF (Depth of Field) photography ^[6].

It does not damage or void the warranty of your camera in any way, since it is just a firmware add-on. ^[7] There are quite a few tutorials on the wiki to put the CHDK on your Canon camera. In brief, here is a short tutorial.

1. Remove your SD card from your camera.
2. Connect the card to your computer (using a card reader) and format it – make sure you backup your old photos before this step! A better way is to use Card Tricks ^[8], format the card as FAT and make it bootable.
3. Download the ZIP file for your camera from this page ^[9] and extract the contents to root folder of your SD card.
4. Safely remove the SD card, and lock it for write-protection this is an important step. The card must be write-protected, or CHDK will not run. This is a simple way to turn on, and turn off CHDK: remove write protection to use the default firmware, and lock it for writing to use CHDK.
5. Enjoy your new firmware, but make sure you read the user manual ^[10] (check which version you want); CHDK is daunting and confusing at first.

For older models, such as Canon A430 (my camera, basically!), download the DISKBOOT.BIN file linked in this forum post ^[11]. Both the pictures shown in this post were taken/modified with my Canon A430.