Note that as a speculative fiction writer, a number of these resources are targeted at speculative fiction writers, but most are of use to all writers.
Writers Digest (a ton of advice and articles on writing, editing, finding an agent, publishing, etc.)
Absolute Write (generally useful site for aspiring writers)
Writers Beware (SFWA site that provides valuable info on agents, editors, publishers and practices that writers may want to avoid)
Predators and Editors (site that calls out bad agents and editors etc.)
Locus Magazine Online (Locus is the premier magazine covering the world of speculative fiction)
Writers and Illustrators of the Future (premier contest for emerging Spec Fic writers and artists)
Tips for newly published authors to help with book tours etcetera from Mary Robinette Kowal.
FINDING FELLOW WRITERS TO EXCHANGE FEEDBACK AND SUPPORT
Meetup.com (search for Writers or Writing groups near you)
Online Writers Workshop for Science Fiction and Fantasy (OWW is a good online critique exchange site for genre writers, with a small annual fee after a trial month and a good mix of pro writers and editors with new writers on it)
Critters (another online critique exchange site, this one free (runs on donations). I preferred OWW but ymmv).
Codex Writer’s Group (online forum for writers who have had professional sales or attended invitation workshops like the Clarions, etc.)
FINDING MARKETS FOR YOUR SHORT FICTION
Ralan (online site listing fantasy/scifi/horror markets)
Submission Grinder (a free submission database alternative to Duotrope that grew after Duotrope began charging for their services)
Duotrope (about $50 a year subscription, searchable market database and submission tracker)
Writer’s Market (about $40 a year subscription. I used to buy the book in the old days. Still a decent resource especially if interested in more than short story markets)
GETTING AN AGENT
Agent Query (a free database of agents)
Query Shark (an agent breaks down what works and doesn’t about queries she gets)
Miss Snark (archived posts snarking on queries and other things useful to writers looking to get an agent)
SFWA (Science Fiction and Fantasy Writers of America, the website has useful resources free to all, as well as membership benefits for paying members)
RWA (Romance Writers of America, joining gives you access to many resources useful for writers in general)
SFF.net (a website with a database of specfic authors and editors, resources, etc.)
Clarion West (Seattle six-week residential speculative fiction writing workshop where you and 17 fellow students write a story a week, critique them, and learn from a different pro each week. They also offer one day workshops and other resources.)
Clarion San Diego (another Clarion-style six-week residential workshop where you and 17 fellow students write a story a week, critique them, and learn from a different pros)
Odyssey (a popular non-Clarion six week workshop, as well as offering a number of online workshops etc.)
Viable Paradise (one week intensive writer workshop in New England)
Taos Toolbox (two week masterclass in writing spec fic that focuses on novels rather than stories. Aimed at writers who have already had some pro sales or attended the other invitation workshops)
Milford (a UK writing workshop, one week in Wales)
You can also find writing workshops and panels at many Science Fiction/ Fantasy Conventions.
The editing process for a novel as described by Patricia Briggs.