Grants.gov (www.grants.gov) is the most comprehensive source for information on federal funding opportunities and offers a variety of subscription services. This web portal posts federal discretionary grants for all of the 26 federal grant-making agencies.
Other Federal Funding Resources
Federal Business Opportunities (FBO)
Catalog of Federal Domestic Assistance (CFDA)
The single government-wide point-of-entry for federal government procurement opportunities worth more than $25,000
SBIR and STTR http://www.sba.gov/content/research-grants-small-businesses
Links to agency scientific research and development (R&D) grants under the SBIR (Small Business Innovation Research) and the STTR (Small Business Technology Transfer) programs.