OWW, or the Online Writing Workshop (formerly run by publisher Del Rey) is a large community of writers on every point in the path from newbie to many-multi-published. There is a relatively modest fee to participate, and posting for critique requires the author to put in time critiquing, as well.
The Burry Man Writers Center is a worldwide community of writers, and it covers everything – business, chat, events, freelance, fiction, nonfiction, markets, agents, resources – you name it. A friendly site with exhaustive resources.
RITRO.COM (Real Insight Through Raw Opinion) In addition to the site itself, the general writing posting board is nicely active, and covers actual subjects beyond “please review my story.”
TeenLit Primarily focused on improving high school writers’ writing, the writing workshop section is wonderful for people of all ages.
Toasted Cheese Named one of the 101 best websites for writers in 2002, it has everything from chat dates for writers to writing exercises to forums for writers (Just the Place For a Snark: general forum, meet & greet, writing prompts; Chasms and Crags: the writing life: share your writing successes & failures, grammar & technique discussions; and Damage From Hail: suggesions, comments, and tech questions). Oh, and don’t forget their market list! An absolute must-visit.