Grants.gov allows organizations to search postings of grant opportunities from all federal agencies.
Catalog of Federal Domestic Assistance is a good place to look for federal assistance, which can be in the form of grants, loans, consultants, etc.
Use GovSpot to link to GrantsNet, CFDA (see above), and SchoolGrants.
Grants Etc. is a website designed and maintained by Professor Armand Lauffer and hosted by the University of Michigan School of Social Work. Provides links to funding sources.
Grants for Nonprofits is a webpage provided by the Michigan State University Library. It is a collection of funding sources, arranged by subject.
GrantsWeb is a huge collection of links to grant information, much of it federal, geared primarily to institutions.
TGCI, the Grantsmanship Center, offers links to grants including international funders. Search by geographic region.