Science Fiction and Fantasy Writers of America, Inc. was founded in 1965 by Damon Knight. It is basically an non-profit association of authors, artists and editors of the science fiction and fantasy genre. The SFWA boasts approx. 1,200 members as of 2001.
It also awards the Nebula award each year for the best short story, novelette, novella, and novel.
See also http://www.sfwa.org/.