Below are resources for starting and maintaining your nonprofit organization (a.k.a. NPO, not-for-profit organization, public service organization or 501(c)3 corporation) in compliance with IRS and other laws.  

• Internet Nonprofit Center (INC) 
  Fabulous set of links to a huge number of nonprofit issues, including organization, management, regulation, development (fundraising and such), and general resources.  THIS IS PROBABLY THE ONLY LINK YOU NEED.

Other Resources

• Nonprofit Charitable Organizations by About.com
  Excellent roundup of the major issues for nonprofits.
• National Council of Nonprofit Associations (NCNA) 
  NCNA is a network of 39 state and regional associations with a collective membership of more than 20,000 community nonprofits.
• State Specific Support Organizations
• Nonprofit Risk Management Center 
  Reduces risk for nonprofit volunteers and paid staff. 
• Nonprofit Services Consortium addresses the needs of regional nonprofit organizations by providing access to a continuum of education, training, and technical assistance.
• Guidestar  
  Searchable database of U.S. nonprofit organizations.
• Benton Foundation  
  Mobilizes networks of researchers, practitioners, and trainers to build the communications capacity of nonprofit organizations.