NHANES: National Health and Nutrition Examination Survey from the Centers for Disease Control and Prevention (CDC). Includes questionnaires, datasets and related documentation.
NHIS: National Health Interview Survey from the CDC. Includes questionnaires, datasets and related documentation.
ADD-Health: The national longitudinal study of adolescent to adult health hosted at the University of North Carolina.
NIF: The Neuroscience Information Framework is an offshoot of the National Institutes of Health.
SAMHDA: Substance Abuse and Mental Health Data Archive (SAMHDA) is searchable on the ICPSR database hosted by the University of Michigan.
The Clinical Trials is a registry and results database from the National Institutes of Health.
Data.gov has a lot of different types of datasets, including some that are helpful for Psychology.