Grants.gov
A brand new site which allows organizations to electronically find and apply for competitive grant opportunities from all Federal grant-making agencies. Navigation is excellent.

FirstGov
A gateway site, "your first click to the U.S. Government". Searches 20,000 Web sites.

GovSpot 
Also a gateway site with short cut to federal agencies or state government information.  Check out Must-See sites, and Do You Know?

Federal Gateway
In addition to being a portal site has a user aid entitled "How to Effectively Locate Federal Government Information on the Web"

FedWorld 
A portal site accessing technical and scientific government databases.

The Foundation Center
Lists Government Resource sites with links.
http://fdncenter.org/funders/web_search/web_search.html  Provides a sector search using keywords to link to Government Resources

The FCIC National Contact Center  
The place to start if you have questions about Federal programs, benefits, or services. Has links to Federal Grants and Useful Resources.

U.S. State and Local Gateway
Organizes links to grants, loans, and contracts by subject.

The Grantsmanship Center
A source  information on federal funding with links to websites and links to international funding by geographic region.