U.S. Government data portal: http://data.gov
NASA Open Data: http://data.nasa.gov
Socrata Open Data Sets: http://opendata.socrata.com/
DC Data: http://data.dc.gov
Dept of Health and Human Services Data Sets: http://www.hhs.gov/open/datasets/index.html
CKAN (Comprehensive Knowledge Archive Network): http://ckan.net/