Many nationwide, longitudinal studies make their data available to the public in order to maximize the utility of the massive undertaking. Some require that you register in some way, others are available immediately. Here is a list of some that I have found:

Inter-University Consortium for Political and Social Research
U.S. Bureau of the Census
National Center for Health Statistics
National Center for Education Statistics
National Election Studies
Metro Chicago Information Center
The Health and Retirement Study
National Data Archive on Child Abuse and Neglect
Henry A. Murray Research Archive
Panel Study of Income Dynamics
The Fragile Families and Child Wellbeing Study