I checked several states and counties, and this database is nearly complete. When you need to look up real property information outside your own state (or are answering a question at Google Answers) this is very handy. It saves you from having to navigate a strange state or county web site to find the Assessor's page.
They also have a link to this:Geographic Names Information System (GNIS)- The New U.S. and Territories Query
Here's what they say: The Geographic Names Information System (GNIS), developed by the USGS in cooperation with the U.S. Board on Geographic Names (BGN), contains information for almost 2 million physical and cultural geographic features in the United States and its territories. The Federally recognized name of each feature described in the data base is identified, and references are made to a feature's location by State, county, and geographic coordinates. The GNIS is our Nation's official repository of domestic geographic names information..."