Here are a few things you can do with them!
play a Youtube video
add an entry to the user's calendar
start an image gallery
ease installing an app from the App Store or Android Market
send a predefined short message (premium services)
send a Tweet with your content
visit Google+/Facebook profile
call a phone number
show contact data as a vCard/meCard
show your branding on textiles (T-shirts, bags) and other articles like mugs
show your profile on recommendation services like Qype
use QR codes in blogs (Wordpress etc)
Yea, QR Codes! They are very easy to use and to create. You download a QR Reader to your smartphone and some apps will create a code for you. However I discovered this site that is super easy to use and that will create QR Codes in multiple size options. Just go to goqr.me/ and start making your first QR Code in seconds.
Want to know everything there is about QR Codes? Wikipedia has an excellent article at en.wikipedia.org/wiki/QR_code#… .
I hope you discover a world of uses for them.