Grants.gov

Established in 2002 as governmental resource named the E-Grants Initiative to improve government services to the public, Grants.gov is your source to find and apply for US federal government grants. It has for managing partner the US Department of Health and Human Services.

Grants.gov allows you to check out all grants offered by 26 federal agencies. 
You can search for grant opportunities by category of funding activities and by agency.
After registration on the website, you will be able to apply on-line for a grant and track the status of your application submission. 
You can also receive by e-mail notifications of new grant postings.

Furthermore, several tools and documents are available to help you with this service.