I made a custom QR code for my site, and one for a local business. It's my first and second attempts at modding QR codes and the second one reminds me of old video games. Very old.
QR codes are bar codes that you can scan with a free smart phone app or specialized barcode reader. They can be used to encode all kinds of information, but they are commonly used as real world "links" to networked services like websites and phone numbers. They can also encode plain text or structured data! It's pretty complicated, but the data is in the pattern, not the colours. The colours, and to a certain extent the shapes, can be modified. The only real requirement is that certain sections of the pattern have to maintain a specific contrast difference in order to be readable by the software.
I made the following barcodes with qrencode and GIMP. Let me know what you think (and what device you tested them with) in the comments below, and drop off links to your own creations if you do something cool!