The Science Fiction & Fantasy Writers of America is a well-known organization of science fiction and fantasy writers.
SFWA hosts the Nebula Awards, aids their members, and runs Writer's Beware.
From their about page:
Today, SFWA is home to over 1800 authors, artists, and allied professionals, and is widely recognized as one of the most effective non-profit writers’ organizations in existence. Each year the organization presents the prestigious Nebula Awards® for the year’s best literary and dramatic works of speculative fiction.
Science Fiction & Fantasy Writers of America