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Quick Overview of the geographic areas covered in the 2007 ACS
Data for Counties and Incorporated Places with populations of 20,000 is available for the first time in the 2007 ACS. These areas will have data based on a 3-year estimate (2005-2007). Larger communities (65,000+) will have data based on both 1-year estimates (2005, 2006, 2007) as well as a 3-year estimate (2005-2007). The data in these excel files is sorted by state and population size (descending), with columns for the Census 2000 population size and then estimates for 2005, 2006, and 2007.

The Reference Resources for Understanding Census Bureau Geography page provides a variety of glossaries, diagrams and manuals that explain geographic areas and faciliate manipulation of data. Below are some highlighted resources::

Geographic Areas Reference Manual
American National Standards Instituted (ANSI) Codes (formerly FIPS)
Current and historical definitions of metropolitan areas. Also, check out the guide to the 'new' metropolitan and micropolitan definitions, "Tracking Metropolitan Amerian into the 21st Century," by William H. Frey, Jill H. Wilson, Alan Berube, and Audrey Singer, published by the Bookings Institute.
The Significant Changes to Counties and County Equivalents listing contains significant county changes only. This includes all county boundary changes affecting at least an estimated population of 200 plus with additional changes of at least one square mile where no estimated population was provided and research indicated that the affected population may have been 200 people or more or "large" annexations of unpopulated territory (10 square miles of more.)

PUMA Equivalency Files [Excel] [Text] (based on Census 2000) describe how each PUMA corresponds to counties in the US.
Mable/Geocorr Correspondence Engine reveals relationships between geographic layers in census geography through a web application at the Missouri Census Data Center
The 1998~FIPS 55-DC3 Index provides a crosswalk between different units of geography. If you can't do something in Mable/Geocorr try this and vice versa.