Critters.org is for serious authors, artists, and creators in any field who wish to improve their craft — those who seek to gain professional stature within their field or increase it.
Other Worlds Writers’ Workshop is for speculative fiction writers only. Spec Fi includes “hard” SF, fantasy, alternate history with elements of SF and fantasy,
Hatrack River Writers Workshop provides a free forum for discussions, writing classes.
Codex brings together pro-level speculative fiction writers in the early stages of their careers who are actively writing. It was created to promote exchange of information, ideas, and writing wisdom among pro-level writers.