U.S. Census Bureau is the main U.S. Census web site containing data sets and lots of information about the census.
http://www.census.gov/

Fedstats is a website which contains data sets and statistical information about federal agencies and departments.
http://www.fedstats.gov/

State Census Data is a website which contains links to state level census agencies, and regional data sets.
http://www.census.gov/sdc/www/

Internation Census Data is a website which contains links foriegn country's census agencies, and data sets from other countries.
http://www.census.gov/main/www/stat_int.html

Census 2000 is an independent website which seeks to suppliment the 2000 census information, and reference the latest census data. Contains a discussion of census elements, and the siginificance of individual demographic elements.
http://www.sunspace.com/cen2000.htm

Census Reference is a website which contains links to various census related data sets.
http://www.virtualref.com/subj/22.htm