In Beta: Google Dataset search: https://toolbox.google.com/datasetsearch
Centers for Disease Control (CDC) Wonder: http://wonder.cdc.gov/datasets.html
HealthData.gov: https://healthdata.gov/browse
Inter-University Consortium for Political and Social Research: http://www.icpsr.umich.edu/
National Bureau of Economic Research, Data: http://www.nber.org/data/