GuideStar.org: www.guidestar.org. Excellent general resources website for nonprofits. Includes advice on what grant makers want to know from applicants: http://www.guidestar.org/news/features/grantadvice.jsp
The Chronicle of Philanthropy: http://www.philanthropy.com/. Latest news and up to date information on fundraising, philanthropy, top donors, foundations, etc. for the nonprofit audience.
PNN Online: www.pnnonline.org. Nonprofit news and information resource. Includes a section on general fundraising.
Government Sources
www.grants.gov. The one-stop site for all government grants.
Catalog of Federal Domestic Assistance: www.cfda.gov. Lists sources for loans, research grants, donations and technical assistance for writing grants.
US Department of Education: http://www.ed.gov/fund/grant/find/edlite-forecast.html. This website is a forecast of funding opportunities under the Department of Education discretionary grant programs for the next fiscal year.
Community of Science: www.cos.com. Described as the "largest, most comprehensive database of available funding", COS funding opportunities include "Monies available for work in all disciplines physical sciences, social sciences, life sciences, health & medicine, arts & humanities."
The Chronicle of Higher Education: http://chronicle.com. Additional resource on funding institutions available on this website at: http://chronicle.com/free/grants/