The free edition of the Foundation Directory Online is good for finding tax returns of known foundation. You cannot effectively search for foundations, other than by geography. However, you can get the last three years of tax returns. You may need to provide your email address to access.
FundRef, the funder identification service from CrossRef, is now available for publishers to contribute funding data and for retrieval of that information. FundRef data includes the name of the funder and a grant or award number.
Contains descriptions of and financial data on over 860,000 nonprofit organization. Information is compiled from tax returns filed with the Internal Revenue Service, and includes each charity's assets, liabilities, revenue, and expenditures. Provides access to 990-PF tax returns, which itemize grants given by private foundations. Maintained by Philanthropic Research, a nonprofit group in Williamsburg, Virginia.