Idealist
http://www.idealist.org/

Bridgestar
https://www.bridgestar.org

Guidestar
http://www.guidestar.org/

Hauser Center for Nonprofit Leadership
http://www.ksghauser.harvard.edu/

Alliance for Nonprofit Management
http://www.allianceonline.org/

Foundation Center
http://lnps.fdncenter.org/

The Chronicle of Philanthropy
http://www.philanthropy.com/

Nonprofit Gateway – Federal Government
http://www.firstgov.gov/Business/Nonprofit.shtml

For entrepreneurial businesses:

The Kauffman Foundation for Entrepreneurial Leadership
http://www.eventuring.org

Gazelles
http://www.philanthropy.com/

Topgrading
http://www.topgrading.com/

Great Game of Business
http://www.greatgame.com/

MIT Entrepreneurship Center
http://entrepreneurship.mit.edu/

Stanford Social Innovation Review
http://www.ssireview.org/