American FactFinder - U.S. Census Bureau

Find here the most current data sets from Census 2000, as well as access to other national, state, and local data.
http://factfinder.census.gov/

American Community Survey

This survey is intended to replace the long form in the 2010 Census. It will collect data from a sample of 3 million households each year, from every county, American Indian and Native Alaska areas, as well as Hawaiian Homelands and Puerto Rico. If implemented (still pending Congressional funding) this survey will be updated every year, providing demographic, social, economic, and housing profiles for areas and subgroups with 65,000 or more people. Smaller areas will be updated less often, but will be updated every 3 to 5 years. This site is still being developed. Not all data is accessible, and not all counties are represented. This will be a site to watch in the future.
http://factfinder.census.gov/home/en/acsdata.html

NEW UPDATED VERSION!
2004 KIDS COUNT Data Book Online

Data from the 2004 KIDS COUNT Data Book are now available in an easy-to-use, powerful online database that allows you to generate custom graphs, maps, ranked lists, and state-by-state profiles; or, download the entire data set as delimited text files. You can also read the book online or view it in Acrobat PDF format. Print copies of the book are available and may be ordered on the website.
http://www.aecf.org/kidscount/databook/

KIDS COUNT Census Data Online - from the Annie E. Casey Foundation

From the Annie E. Casey Foundation, KIDS COUNT has compiled indicators of child well-being from the 2000 U.S. Census and this site provides access to an interactive online database of that Census data. As additional data becomes available, KIDS COUNT will include it in their database. You will have access to profiles, rankings, and raw data for the nation as a whole, for individual states, the largest cities, counties, and other geographic areas.
http://www.aecf.org/kidscount/census/

NEW!
2003 Youth Risk Behavior Survey (YRBS)

 

The YRBSS includes national, state, and local school-based surveys of representative samples of 9th through 12th grade students. These surveys are conducted every two years, usually during the spring semester. The national survey, conducted by CDC, provides data representative of high school students in public and private schools in the United States. The state and local surveys, conducted by departments of health and education, provide data representative of the state or local school district.
http://www.cdc.gov/HealthyYouth/yrbs/