The Data.gov Mash-a-thon took place in Washington, D.C. at the end of August 2010. The event was designed to inspire developers to find new ways of using Data.gov’s free public data to create mash-ups. One of the most interesting mash-ups developed was the Energy Data Mash-up which created a regional map of 7 U.S. cities using energy information from Data.gov. the Energy Data Mash-up is available at http://en.openei.org/apps/mashathon2010/ and offers an interesting comparison of census, utility usage, and smartgrid information in cities with populations around five hundred thousand.
Participants in the Mash-a-thon include Rensselaer Polytechnic Institute Professor Jim Hendler and his students who have already created more the 40 different Mash-ups using Data.gov information. A list of these Mash-ups is available at http://data-gov.tw.rpi.edu/wiki/Demos and is useful not just for the mash-ups, but also in seeing what information and technology was used to put the mash-ups together.
More information about linked data and mash-ups, including tutorials can be found at http://linkeddata.org/