Qualified tax-exempt organizations are listed by state in the SOI Tax Stats - Exempt Organizations Business Master File Extract (EO BMF). To double check an organization's status, call the IRS at 1-877-829-5500.
You can look up tax-exempt organizations qualified to receive tax-deductible contributions using the Exempt Organizations Select Check. Note: There is a delay of up to a month before new/reinstated organizations get on the list. Again, you can double check an organization's status by calling the IRS at 1-877-829-5500.
Until 2012, qualified tax-exempt organizations qualified to receive tax-deductible contributions were listed in Publication 78, Cumulative List of Organizations. The list was available on the IRS website and published in print each September, with supplements listing new additions (but not revocations) published quarterly. To find out more, you can call the IRS (877-829-5500) or search the Internal Revenue Bulletins to find the notices announcing additions and revocations to Pub 78 (see entry for "Internal Revenue Service").
IRS exemption rulings are on Westlaw (FTX-EXEMPT).
IRS guidelines on charitable contributions are published in IRS Publication 526. Pub 526 is posted on the IRS Web site.
Tax-Exempt Org news is reported in the monthly Exempt Organization Tax Review by Tax Analysts.
Donors: Noza Search lets you look up individuals who and institutions that make charitable donations.
Tax returns (IRS Form 990s) for most tax-exempt organizations are posted by Guidestar, Noza and the Foundation Center. Exceptions include religious organizations, Federally chartered organizations and small charities with annual revenue less than $25,000. The form may also be posted on the organization's Web site. If that doesn't work, try contacting the organization, the Foundation Center (for returns from the past few years) or file Form 4506-A with the IRS. Also, you can get pre-1996 Form 990s from the Indiana University Purdue University Indianapolis (IUPUI) University Library (317-274-0464), which holds the Foundation Center's Historical Foundation Collection.
Smaller tax-exempt organizations are allowed to file a short Form 990-N nicknamed a "postcard." You can search for and retrieve the information on postcards using the IRS Exempt Organizations Select Check with the "Have filed Form 990-N (e-Postcard)" button checked.
Churches are automatically tax-exempt; they do not have to file anything or register with the IRS, although some do anyway (see Pub 1828, Tax Guide for Churches and Religious Organizations).
Individual Organizations: To get more information about individual tax-exempt organizations, try the National Center for Charitable Statistics and/or click on the cross-reference to "Foundation" and "Non-Profit Organizations," below.