In addition to the resources below, the nonprofit organization's website is a valuable source of information and may have more recent IRS Form 990s.
GuideStar (requires free registration)
GuideStar provides 990s (the IRS filing for most nonprofits), which contain the nonprofit’s financial information, including compensation and major services’ expenses. The website also has some nonprofit profiles.
Charity Navigator (open access for ratings, requires registration to access 990s and some other information)
Charity Navigator rates charities (not foundations) for financial health and transparency/accountability, based on IRS Form 990s and a review of the organization's website. Its database includes only US-based charities whose revenue exceeded $1M in the past two years. The publicly-available information includes ratings, the most recently-available year's financial information, and basic information on the charity.
Foundation Directory Online (Temple University access)
This database contains information on grantmakers. Information available includes the organization's background, program areas, financial assets and giving, online presence, and staff, with links to 990s going back as far as 2000. Searchable by organization name or topical or geographic area(s) of interest.
Candid (open access)
Candid (former associated with Glass Pockets) has a website providing 990s for private foundations, public charities, and nonprofits. Coverage dates are spotty.
Nonprofit Explorer (open access)
Nonprofit Explorer is a ProPublica website containing 990s. Its archive extends back to 2001, further than many other websites' and databases' materials.
National Directory of Nonprofit Organizations (Temple University access)
This is a Gale directory with basic information about nonprofits, including ruling date, assets, income, and contact information. Temple Libraries provide access to the 2008 edition. Can be searched by organization name, geographic location, or description. The Encyclopedia of Associations (covering nonprofit associations), last edited in 2018, also can be accessed via this link.