Don't let the name fool you, this is not just for sciences. The Community of Science is one of the largest grant search engines which covers all fields of interest. Search engine may be used to look for grant opportunities or collaborators (COS Expertise).
This is the largest database of corporate and private foundations in the United States.
This search engine is maintained by the Grant Resource Center which is a division of American Association of State Colleges and Universities (AASCU). This site also provides other useful information such as tips on proposal preparation and email updates on funding opportunities.
Indiana Youth Institute: Good source for grants opportunities involving children or family initiatives. Sign-up for their weekly electronic newsletter which has a grant tip each week.
Grants.gov: This is the main site for all federal grants. It is not useful as a general search engine, but it can be helpful to find a link to a specific agency of grant opportunity.
Yahoo Directory: This search engine is more targeted than the general Yahoo search engine. It is useful in finding professional organizations that often have small grant programs for their specific area of interest.
Zyn Systems: This is a clearinghouse website for information on Small Business Innovation Research (SBIR) Opportunities.