As 640K posted, you can use the Barcode Scanner app to create a QR code right on your droid's screen for someone else to scan. To create codes that you can save, post on the web, etc, use one of the generators from Mr. Steve's link.
You can order a customized "add to friends" T-shirt which will let someone scan your shirt and automatically add you on Facebook. You could also print QR codes on marketing materials like fliers or ads, on a coffee mug, etc. Basically, any time you want to make electronic information available to someone who isn't at their computer.