QR tags (square or 2D barcodes that can hold up to 4,296 characters) are famously 'big in Japan'. Outside of Japan they've often seemed a solution in search of a problem, but we're getting closer to recognising the situations where they could be useful.
There's a great idea in this blog post, Video Print:
By placing something like a QR code in the margin text at the point you want the reader to watch the video, you can provide an easy way of grabbing the video URL, and let the reader use a device that's likely to be at hand to view the video with...I would use this a lot myself - my laptop usually lives on my desk, but that's not where I tend to read print media, so in the past I've ripped URLs out of articles or taken a photo on my phone to remind myself to look at them later, but I never get around to it. But since I always have my phone with me I'd happily snap a QR code (the Nokia barcode software is usually hidden a few menus down, but it's worth digging out because it works incredibly well and makes a cool noise when it snaps onto a tag) and use the home wifi connection to view a video or an extended text online.
As a 'call to action' a QR tag may work better than a printed URL because it saves typing in a URL on a mobile keyboard.
QR tags would also work well as physical world hyperlinks, providing a visible sign that information about a particular location is available online or as a short piece of text encoded in the QR tag. They could work as well for a guerrilla campaign to make contested or forgotten histories visible again - stickers are easy to produce and can be replaced if they weather - as for official projects to take cultural heritage content outside the walls of the museum.
The Powerhouse Museum have also experimented with QR tags, creating special offer vouchers.
Here's the obligatory sample QR - if your phone has a barcode reader you should get the URL of this blog*:
* which is totally not optimised for mobile reading as the main pages tend to be quite long but it works ok over wifi broadband.
[Update - I just came across this post about Barcode wikipedia that suggests: "People would be able to access the info by entering/scanning the barcode number. The kind of information that would be stored against the product would be things like reviews, manufacturing conditions, news stories about the product/manufacturer, farm subsidies paid to the manufacturer etc." I'm a bit (ok, a lot) of a hippie and check product labels before I buy - I love this idea because it's like a version of the ethical shopping guide small enough to fit inside my wap phone.]
[Update 2 - more discussion of a 'what are QR codes good for' ilk over at http://blog.paulwalk.net/2008/10/24/quite-resourceful/]