These are some websites and communities that I've found helpful as a writer. If you have any suggestions that would make worthy additions to the list, feel free to let me know.
Science Fiction & Fantasy Writers of America
SFWA is a professional organization for authors of speculative fiction. Its stated mission is to inform, support, promote, defend and advocate for its writer members. They host the prestigious Nebula Awards, assist members in legal disputes with publishers, and administer benevolent funds for authors facing medical or legal expenses. They have specific criteria for membership, but even non-members can benefit from their Information Center and Writer Beware (which is linked below).
Codex Writers' Group
Codex is a group for "neo-pro" speculative fiction writers, featuring an active forum, chat room, critique system, and more. It's an amazing place for learning, exchanging ideas, and meeting other authors at similar points in their career. It's free to join, but they do have some criteria for membership eligibility.
An informational site for writers of every kind. Be sure to check out the AW Water Cooler, a giant forum with lots of advice from great people if you're willing to grab a shovel and dig in.
The (Submission) Grinder
The Grinder is a free submission tracker and market database for writers of fiction (with non-fiction and poetry somewhere on the horizon as well). As a short fiction writer looking to sell and publish my work, this site is immensely useful, especially in the wake of Duotrope's move to a subscription model.
An enormous database of speculative fiction markets. If they're buying SFF stories, chances are you'll find them listed here.
A publishing industry watchdog group sponsored by SFWA. The Writer Beware Blog is essential reading for anyone looking to stay one step ahead of the predators and scam artists who exploit hungry writers new to publishing.
A lesson in writing query letters, via the wholesale (and often hilarious) dismantle of real-world examples. Note: all flagellation is completely voluntary.
A neat tool that lets you see if anyone online is plagiarizing your work. Useful for bloggers, writers, and authors alike.
A catalog of popular (and not so popular, in some cases) fiction conventions. Just about every trope you've ever come across in literature, film, or television has a home here. Beware! This site is a time vortex!
National Novel Writing Month is an annual writing blitz designed to get writers writing. Every November the community springs to life with the goal of writing 50,000 words before the end of the month. This is also a great way to meet writers in your local community.
Free software designed by Simon Hayes, tons of which is geared toward writers. My favorite is "Sonar," a submission tracker, which is especially useful in the wake of Duotrope's move to a subscription model.