They make it so much easier to share (and find) structured information. Tantek Celik is one of the leaders of the cause; you can find out more at the microformats website. It all began (or started coming together, anyway) at Foo Camp the summer before last, when a bunch of calendar guys were sitting around trying to figure out how to make scheduling data easier to mark and find on the Web. Now it extends to hCards as well as hCal, and many more. The one I want is a way to mark expenses so that they can feed directly from a calendar into an expense-account form.