Foundation Center - the Foundation Center provides information about very foundation in the country. Much of this information is free, but some of it is for paying subscribers only.
Grants.gov - Grants.gov is your source to FIND and APPLY for federal government grants. Learn more about Grants.gov and determine if you are eligible for grant opportunities offered on this site.
Guidestar - Guidestar lists financial information for 800,000 nonprofits. You can use Guidestar for research, and you can also update your organization's entry there.
Michael Gilbert - maintains a great site with daily news updates bridging the online and nonprofit worlds. You can read the news there, or you you can subscribe for weekly email updates.
The Center for Philanthropy and Nonprofit Leadership - is building a resource center with lots of great information for starting, running and funding an organization.
Putnam Barber - Putnam Barber maintains the Nonprofit FAQ an older resource center that is special because it is based on actual questions and answers posted by nonprofit managers over the years.
Network for Good - find volunteer opportunities, make donations to hundreds of thousands of nonprofits. Plus, you can use this site to add a free Donate button to your Web site.
Service Leader - a huge site covering every aspect of volunteer management, from screening, matching, record-keeping and evaluation, to legal issues, risk management and virtual volunteering.
Techsoup - Techsoup has built the leading resource center for all issues related to nonprofit technology. In addition, they offer steep discounts on software for nonprofits, sometimes up to 90% off the list price.
Volunteer Match people can find volunteer opportunities all over the country by ZIP code or area of interest.
Idealist - has built the leading nonprofit job site in the country. In addition to jobs, you can also list events, volunteer opportunities and internships.
Other Links
National Center for Nonprofit Boards
The National Council of Nonprofit Associations