"The purpose of Data.gov is to increase public access to high value, machine readable datasets generated by the Executive Branch of the Federal Government. Data.gov enables the public to participate in government by providing downloadable Federal datasets to build applications, conduct analyses, and perform research."
Data.gov offers access to a large and ever-growing compilation of data sets, tool, and more. The site is divided into:
"Raw" Data Catalog, where you can search for data by keyword, file type, federal agency, or category.
Tool Catalog, containing "simple, application-driven access to Federal data with 
hyperlinks. This catalog features widgets, data mining and extraction tools,
applications, and other services"
Geodata Catalog, with links to download geospatial datasets and a metadata page with details on the datasets, as well as links to more detailed Federal Geographic Data Committee (FGDC) metadata information.
Folks are taking these data and creating all kinds of mashups and other tools. Check out apps like Local Recovery (how much Recovery Funding has gone out in your current location), Superfund Finder (discover more about the nationwide distribution of Superfund sites as well as sites near you, or Geodata Explorer (get a true sense of the economic situation at the city or block level).
