http://www.asmp.org - American Society of Media Photographers. Find a photographer.
http://www.nanpa.org - North American Nature Photography Association also promotes nature appreciation and environmental awareness.
http://www.nppa.org - National Press Photographers Association. Articles and resources on ethics, copyright, billing practices, and current issues.
http://www.photoshopuser.com - For members of the National Association of Photoshop Users (NAPP). Costs $99/year. Not so much an association as it is a magazine and excellent online reference.
http://www.ppa.com - Professional Photographers of America. Find a Photographer, business services, calendar of events, classifieds, vendors directory, press releases, chats, message boards.
http://www.psa-photo.org - The Photographic Society of America (PSA) is a 74 year old association of advanced amateur and professional photographers with over 5000 members in the US and 60 other countries. They have a week long conference each year, and publish a magazine, the PSA Journal.
http://www.rps.org - The Royal Photographic Society has history, classes, message boards, and articles.
http://www.wppinow.com - Home of the Wedding and Portrait Photographers International. Offers a referral service that is said to be linked to Martha Stewart's website, but I could not find the link. Also sponsors tradeshows, Rangefinder magazine, newsletters, competitions.
http://www.yahoo.com/Arts/Visual_Arts/Photography/Organizations/ - This is a list of some of the major photography organizations, with links to digital picture archives, holography, and more.