QR Codes are a useful way of getting data to your mobile phone using it’s built-in camera. They’re basically 2D barcodes that can carry things like URLs and contact details. On Android I’d recommend using the app called Barcode Scanner to read QR Codes.
There were instances where I wanted to open up a webpage with a long URL on my phone & didn’t want to go through the hassle of copying and pasting it into one of those online QR Code generators.
This bookmarklet for Firefox and newer versions of IE lets you create a QR Code for the page you’re on.. it’ll default to the current page, but you can also override this with something else (like a URL from your clipboard, or whatever).