A QR Code is a type of two-dimensional bar code which most new camera phones in Japan can read with their camera. It's used for URLs and other information, and is particularly useful for URLs in flyers, catalogues, and other printed material. The QR Code was invented by Denso, who have a QR Code home page.
Docomo, AU and Vodafone phones all read a plain URL in QR code as a link that you can jump to. However, each vendor supports some additional functionality based on how the text in the QR code is formatted. Docomo and Vodafone both support e-mail shortcuts, contact information, and links that can be automatically bookmarked with a title.
Developer Info
Google has a good overview on their chart api page
A blog post about creating custom QR codes
QR Codeのオススメ (Japanese only) contains a lot of information about QR codes on Japanese keitai, including handset shots.
A Neography blog entry has a bunch of links.
Generating QRcodes
Google: http://chart.apis.google.com/chart?chs=140x140&cht=qr&chl=<url>
Swetake has a bunch of different QR code generators available, including Perl, PHP, ruby, and Java versions, and even an i-Appli. There's an English version of the page as well, but it is not as up to date, and had only the PHP and perl versions.
Generating QRcodes UIs
QR Codeのススメ (Japanese only) contains a lot of information about QR codes on Japanese keitai, including handset shots.
NFG on-line generator, which knows about some of the special formatting used by the various phone manufacturers. This appears to be using code from Swetake above.
Kyle Barrow's Codatron.