Photographer Maciej Pietuszynski has posted a (hopefully tongue-in-cheek) step-by-step tutorial over on his blog on how he was able to give his Canon 40D a square aspect ratio viewfinder by applying some tape to his focusing screen.
It could be a funny prank for convincing people that you have some kind of special, limited-edition DSLR, but be warned: focusing screens are extremely susceptible to dust and scratches. Unless you want to risk messing up the viewfinder of your camera, you might want to refrain from doing this mod yourself.
Change The View [maciekpp via DIYPhotography]
Photographer Adrian Onsen wanted to use the AI Servo autofocus mode on his Canon 40D in low-light situations, but found that the AF assist beam is only emitted once until focus is achieved rather than every time the camera needs to refocus. He then purchased a laser pointer from a dollar store, disassembled it to obtain a defocused beam of light, and attached it to the top of his camera. The hacked-together AF assist tool ended up working pretty well — Onsen was able to shoot sharper photos at a dance club without anyone noticing the extra light. To learn more check out his in-depth writeup here.
AF Assist tool (via Hack a Day)
Canon published this humorous advertisement for the 40D in Digital Photographer magazine back in 2008.