Commonwealth Writers
http://www.commonwealthwriters.org/

National Writers Union (represents writers working in print, electronic, and multimedia formats)
https://nwu.org/

Science Fiction and Fantasy Writers of America
http://www.sfwa.org/

Society for Technical Communication
http://stc.org/

Writers Guild of America, West (labour union for motion picture, broadcast, cable and new media industries)
http://www.wga.org/