The Open Geospatial Consortium just announced that KML has been adopted as an open standard. KML stands for Keyhole Markup Language, originally designed by Keyhole who was acquired by Google back in 2004. Keyhole's Earth Viewer product was reborn as what we know today as Google Earth. I first came across KML when I was working on some data visualization using Google Earth. I found it to be very expressive and easy to use. You can do neat things like stream dynamic KML to animate the map or add overlays on the map.
It's interesting to see how Google Earth/Maps's popularity has allowed KML to become the international standard. It will be even more interesting to see how quickly the standard is adopted by many of the geo-visualization products out there. While I don't think it will be "the HTML of geographic content", I do think this standardization will open up the market for new products that build/support KML, pushing KML to its limits as we have done with HTML.