2010 U.S. Census data is out! So is of course, the Census 2000 data, and each year's American Community Survey (ACS) results since 1995. There has never been more data, or data more accessible, from the good Bureau than it is now.
The official interface to Census data is embedded to the right. Here are more useful links to Census and related resources:
American Fact Finder web page:
Census TIGER GIS data products:
All about ACS in GIS by Lynn Wombold, Esri Chief Demographer:
U.S. Census home page:
Global information from the Population Reference Bureau: