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
A highly respected professional organization for authors of speculative fiction. Even if you're not a member, their website is a great resource.
An enormous database of speculative fiction markets. 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 informational site for writers of every kind. Be sure to check out their forums, the AW Water Cooler, which has an insane amount of advice from great people if you're willing to grab a shovel and dig in.
A publishing industry watchdog group sponsored by the SFWA. Essential reading for anyone looking to stay one step ahead of the predators and scam artists looking to 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.
The Bookshelf Muse
An awesome blog filled with great resources and tools for writers, including an Emotion Thesaurus, Setting Thesaurus, and Character Traits Thesaurus.
A neat tool that lets you see if anyone online is plagiarizing your work. Useful for bloggers, writers, and authors alike.
A catalog of the tricks of the trade for writing fiction. 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.
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.