A special-edition of Jules Verne's Around The World In 80 Days has been published with QR codes included at various points of the book. Pointing an iPhone (loaded with Ubimark's app), the codes take you to relevant sites for context.
It's like the 21st century version of footnotes, I guess you could say. When you turn the page and see a QR code, the app recognises it, and whizzes you off to a website for sidenotes, historical information and even discussions about that particular section.
A couple of years ago, a British newspaper was redesigned and now includes links at the end of many of their articles, pointing to longer versions on their website. This was deemed pretty revolutionary for the print industry, but QR codes in a 137-year-old book? Brilliant.