USA.gov - This government site will connect you to a number of valuable federal resources on the local and national level.
Candid - The searchable database includes the mission, programs, finances, and contacts for over one million nonprofit organizations in the United States.
National Council of Nonprofits - This is the largest national network of nonprofits and an excellent source for resources and advocacy.
Free Management Library - Collection of free resources about personal, professional and organizational development, serving people in businesses, nonprofits and universities. It offers more than 10,000 resources -- books, links, articles and more recently blogs. Be sure to check their Catalog of Nonprofit Literature.
Foundation Center Proposal Writing Short Course (Candid) - A fantastic resource for preparing a proposal.
Idealist - Excellent job & internship hunting site for those interested in nonprofit work.
The Chronicle of Philanthropy - A trusted source for news, jobs, and hot topics in nonprofit and philanthropy work.
The NonProfit Times - Another trusted site for news, grants, and resources related to nonprofits.