Associations Unlimited
Associations Unlimited is a database of nonprofit organizations and national associations from across the world.

Guidestar
http://www.guidestar.org/
Guidestar allows users to search for information about nonprofits and in some cases obtain their IRS Form 990s.

Foundation Directory
Foundation Directory provides access to timely, comprehensive information on grantmakers and their grants. Included is a database of the entire universe of over 100,000 foundations, corporate giving programs, and grantmaking public charities in the U.S.; a database of over 4,000 sponsoring companies, offering a quick pathway to corporate funders; a database of over 2.4 million recently awarded grants; and a keyword-searchable database of over 700,000 recently filed IRS Forms 990 and 990-PF.