Writersmarket.com Organizations - An alphabetical list of selected writers' organizations. This list is easier to navigate than other WM databases since it's only six screens. However, it is not comprehensive; for instance, the Maryland Writers' Association is not on its list.
Local Writers' Associations and Groups by State - This page offers a comprehensive list of state and regional writers' organizations and associations, but [WARNING] it's on Squidoo, which means it's a web page maintained by an individual and not an organization, so it may disappear or be altered at any given time.
Poets and Writers Literary Organizations -A short/selected list of major writers' organizations.
Individual Organizations/Associations of Note
[Note: Inclusion here does not mean an endorsement on my part; I am just noting their existence/availability.]
American Independent Writers
National Writers Union (NWU) - The only labor union that represents freelance writers.
The International Association of Writers - (Scroll down their home page, and you'll find a list of "Best Writer Organizations," which includes themselves, SCBWI, NWU (see above), and the American Society for Journalists and Authors.)