I have to say that my favorite organization is Romance Writers of America. Mostly because of the local chapters. I belong to Tacoma and Seattle chapters (both in Washington State) as well as National. Actually you have to belong to national in order to join local chapters. This is true of most national writers organizations. I'd enjoy RWA even if I hadn't written romance. The chapter meetings are usually informative and well-attended. I don't get to as many meetings as I used to, now that I live in the back of beyond, but I do try to get to some of the national conferences. One of my most recent was in Dallas. I got to sit in on several police and related topic seminars, and one on "a day in the life of a cowboy," which was excellent for research and a lot of fun besides. I've made some of the best friends of my life through RWA.
This I think is one of the biggest benefits of joining a national organization--the people you meet. RWA doesn't require that you be published, just that you are actively interested in writing romance. More details are available at their website. RWA also puts out a very informative monthly magazine, Romance Writers Report.
Mystery Writers of America also has local chapters. Mine (Seattle) recently disbanded. It's getting very difficult to get people to take office, everyone's too busy. I've joined the Northern California chapter and will get their newsletter, and maybe even attend an occasional meeting. MWA allows non-writer members, again with an active interest in the mystery genre, but they can join only as Associates. Details at the web site.
Local chapters put out a newsletter usually and hold meetings with speakers who are law enforcement officers, pathologists, crime scene specialists and so on. Again, the fellowship is wonderful and worth the price of admission. And of course MWA national awards Edgars annually for those books and short stories, etc. deemed to be the best of the year.
Sisters in Crime exploded on the scene not too many years ago. It's a going concern, with an extremely informative newsletter and interesting chapter meetings. You don't have to be published to join. And you don't have to be female either! There is of course a website.
I'm also a member of American Crime Writers League, which is open to published authors only. And the International Association of Crime Writers, whose title speaks for itself. Again, only open to published writers.
I'm not a member of all the following organizations, but I've brought in links for you to follow.
One feature of most of these organizations is that you can publicize your books in their local newsletters, and in some of the national ones.
Links to the following organizations can be found at Writers and Writing, Community of the Book Orgs.
There are probably more organizations. If you know the URLs of others, please email them to me and I'll add them to the list.