The National Center for Charitable Statistics at the Urban Institute
The national repository of data on the nonprofit sector in the United States.
Public charities and private foundations must file a 990/990-PF with the IRS every year in order to keep their tax exempt status. These filings include an accounting how much the charity raised and how they used their money. It also includes compensation information for the highest paid employees.
Several resources provide access to these filings:
Charity Navigator - must create a free account to see the 990 filings
Foundation Directory - includes foundations, corporate giving programs, grantmakers, and more. Provides access to 990s.
Guidestar - must create a free account to see the 990 filings