http://govinfo.kerr.orst.edu/index.html Government Information Sharing Project at Oregon State. Comprehensive and Easy to Use. The home page lists the available data sets.
http://fisher.lib.virginia.edu/pums Geospatial & Statistical Data Center. ( Students who are looking for data by individual characteristics such as gender and income must use the PUMS data. Data sets are compiled for areas not political subdivisions, although counties with large populations are frequently tabulated as a single data set.)
http://www.ciesin.org/interapps.html Center for Informational Earth Science Information Network. This particular page allows the user to create theme maps using population data with a variety of political and census boundary polygons.
http://www.census.gov U.S. Census Site
http://www.wa.gov/esd/lmea/pubs/profiles/profiles.htm A Washington state site maintained by the division of Labor Market & Economic Analysis. There is very useful county level data here.