There are lots of places on the internet where writers gather: I've written some general posts about them here. Many writers' message boards include a discussion forum and an area where writers can receive feedback and criticism on their work, but some of them are far better than others: it all depends, I think, on what the writers who populate the place are like. If a writers' site has a good proportion of accomplished, well-published writers in its membership then the advice there is likely to be above average; if the members are all aspiring writers with little or no experience of publication, or there's a high proportion of vanity-published writers among the membership, then it's very likely that the advice that's on offer isn't going to be quite so good.
Here are a few of my favourites. I'd be grateful if you'd provide links to the places you like in the comments, and let me know what's so special about them.
AbsoluteWrite is probably the best writers' site on the internet. It's got a fabulous message board and it seems to me that the people who comment there have a higher-than-average level of intelligence and publishing expertise. It's American, and lots of the information it contains is restricted to that continent: but there's still an awful lot of good stuff there for anyone who is interested in writing, revising, and getting published. It's free to read, free to join, and free to take part in, and I love it.
Authonomy is HarperCollins UK's interractive writers' site. There's a sprawling message-board (which takes forever to load on my dial-up service), a large selection of work on view; and every month the five (I think) most well-received pieces of work get looked at by HarperCollins editors, and are commented on. This is not to be sniffed at: but do bear in mind that so far not one of any of the top five books has been published by HarperCollins, and only one book has been taken from the Authonomy slush-pile for publication.
Litopia has a good reputation, and several of my friends are members: however, I've never stuck with it long enough to gain access to its inner workings, and so cannot vouch for its quality (although I'm sure that a few of my readers will be able to advise).
WriteWords is a UK-based site which charges an annual membership fee but which does have a very high proportion of published and successful writers in its members: consequently, the advice that you'll find there is well above average, and very valuable. It's very writing-focussed, with an excellent peer-review system across lots of genres.
YouWriteOn. What can I say? It began with the excellent premise of providing writers with an online community and the chance for the best writers there to get a critique of their work from a professional editor or agent; but then in the autumn of 2008 it introduced a vanity publishing option which was neither well thought-out nor well-received. I really don't like its vanity publishing scheme; and from what I've seen, the standard of writing and advice on its boards are not nearly as high as you'll find on other boards.
Zoetrope has sections for novel-writing, short fiction, screenwriting, and all sorts. It has a slightly confusing system of public message boards with a subculture of private rooms, which any of its members can set up and invite people to join; it's well thought-of, and has a reputation for producing excellent writers through strong criticism and robust comment. I have taken part, but found the system of private rooms (where most of the action goes on) made it a little too scattered and time-consuming for my liking.
Further recommendations from our readers:
The Backspace Forum
The Online Writing Workshop for Science Fiction, Fantasy, and Horror has a mixture of published and yet-to-be published writers. Also the chance to have your work critiqued by well-known authors or editors.
Scrawl: The Writers Asylum has a number of talented and helpful writers in several genres.
The Writer's Chronicle