Since their rise to fame a couple of years ago, we've seen QR codes used in a variety of different ways. One of the most popular uses has been to quickly link someone to a website, either from within a website itself, or via a magazine or newspaper. Some download sites have even begun implementing them to allow people to download a file to their mobile fast, rather than have to manually type out a long, complicated URL.
One of the most creative uses I've personally seen has been with a coffee shop up here in the Great White North, for those looking for work. Instead of making these folks stand in line and disturb a manager who is probably very busy, a QR code on the door links people directly to an online job application.
Until now, that was the coolest QR example I could think of, but after seeing what Hackerspace Charlotte has come up with, it's been superseded. The folks here, along with those at a metal recycling facility called Southern Resources, hopped on top of the business with paint-cans and brushes in-hand, and got to work.
The result is a permanent 100' x 100' (10,000 square feet) QR code that can be seen via Google's satellite. Unfortunately, due to the angle, none of my devices have been able to read the code properly (Edit: As DanielWalkswithstick points out in the comments, unchecking the 45 degree angle option in Google Maps fixes this problem), but it links back to "http://qr.hsclt.org/" - aka, the site that discusses the accomplishment.
Interestingly, it only took a mere 3 weeks from the time the QR code was finished for it to show up in Google's satellite imagery. Given how "easy" this would be to pull off on a flat roof, I can't help but wonder if we'll begin to see more people trying their hand at it as well.