This site provides data by topic and is a good place to start if you're looking for a topic.
Statistical Data Sets
Here you'll find data describing numerous categories. Download tables of graphs of the data.
Statistical Abstract of the United States. Start here for all your domestic statistical needs. I suggest you use the index to find the char(s) you need, keeping in mind that the numbers listed there are chart numbers, not page numbers. Need more detail? Use the URL in the source which is listed below each chart.
FRED at the Federal Reserve Bank of St. Louis
FRED contains numerous statistics about the US economy. Data can be downloaded to Excel.
National Longitudinal Survey of Youth97 (NLSY97)
This longitudinal survey from the Bureau of Labor Statistics provides data about young people and their families.
OECD Health Data
Covers health-related data for member OECD nations. It's available through the ZSR database page.
Agency for HealthCare Research and Quality
from the U.S. Department of Health & Human Services
Health Interactive Data
From the Centers for Disease Control
National Center for Health Statistics
from the Center for Disease Control and Prevention
National Vital Statistics System
go here for data on births, deaths, infant mortality, etc.
State Health Facts
provided by the Kaiser Family Foundation
Statistics from WomensHealth.gov
a division of the U.S. Department of Health & Human Services