There are several, all with different focus. Of the ones I can think of offhand:
AW is probably the most newbie-friendly (both the interface and what you can do here), and gets folks the most direct editing help.
Hatrack only allows short posts for edit/evaluation purposes, but gets into deeper theoretical discussions of writing and its elements than I've seen on AW. The interface is simple but workable; however there's no notification of replies. It tends to be relatively formal and polite as forums go.
Forward Motion probably has a higher concentration of pro-grade SF/F writers, but the interface is a PITA. (I'm used to it from other forums that use the same software, but it's still a nuisance to navigate.)
Writers.net tends to be too snarky for my taste, and the interface is strictly one-post-at-a-time navigation.
Baen's Bar has a submit-and-discuss area. It can be accessed via newsreader, which us old farts think is way cool. Some of Baen's authors have their own subforums there.
At present I generally hit AW and Hatrack every day, and the rest only occasionally.