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Microdata (Wikipedia ref) is one of those HTML5 things that I've been meaning to take a closer look at for some time. At a very high level, you can think of microdata as a way of embedding context into your HTML to provide information to search engine crawlers and browsers. It isn't meant for the human eye, but can help out automated agents viewing your site.
I ran into a great example of this today: Creating a semantic breadcrumb using HTML5 microdata
In this article, Euan Torano describes how he used microdata to provide context to Google. You can see in his screen shot that the microdata was visible in the Google Search Results. Not only that, the same HTML used to provide that microdata could be used as nice breadcrumbs in general.
I'd love to know if any of my readers are making use of this yet.