Grants.gov: Federal Register and links to federal agency Internet sites. Provides notices of federal funding availability, announces grant opportunities, and gives general information about grant programs. Requires enrollment to submit proposals online.
Foundation Center: Provides notices of RFPs issued by foundations, a list of more than 65,000 foundations in the United States and search methods for locating foundations and their Web sites. Will provide free notice of grants available if you enroll. Also provides online message board for fundraisers and grant writers.
Council on Foundations: Provides links to other sites such as Government Grant Sources and Private Sector Foundations. Links to Federal Register, Grants and Related Resources from Michigan State University, Resources for Grant Writers, State Grants, Schrock's Guide for Educators, Yahoo!'s Directory of Education Grants. Includes information about such things as Principles and Practices for Effective Grant-Making, Private Independent Foundations, Public Foundations, upcoming conferences and workshops, and Internet resources for nonprofits.
Fundsnet Online Services: Sources for grants, fundraising, grant writing, and philanthropy online since 1996. Categorized links to other sites.
FundersOnline: Links to European Union institutions, philanthropy, news sites, searchable database of nonprofits, and research information.
Michigan State University Libraries: Links to foundations, grant categories, and other useful information for grant writers and nonprofits.
Idealist.org Nonprofit Information: Internet nonprofit center. News and information for charities and donors.
GuideStar.org: Provides IRS reports for both nonprofit organizations and foundations. Good source for analyzing foundation giving patterns.
Nonprofit Good Practice Guide: Online resource for nonprofit leaders and grant writers regarding all aspects of nonprofit business management.
You'll also find many, many catalogs that list grants for particular kinds of agencies. Regional Associations of Grantmakers often release catalogs of foundations (and sometimes interactive CD's) that can facilitate research into geographically relevant grantmakers.