*Updated January 2020

These are just a sampling of the many great (and some of my favorite) sites out there for writers. 

Must-Visit Writing Sites

Agent Query
Author Marketing Experts (Penny Sansevieri)
Miss Snark (she's retired now, but the site is still packed full of good info)
Miss Snark's Crap-O-Meter Archives (a must for anyone trying to learn how to write query letters)
Mystery Writers of America
Query Shark
Query Tracker
Science Fiction and Fantasy Writers of America (SFWA)
Society of Children's Book Writers and Illustrators (SCBWI)
Southern Breeze (AL, GA, FL Panhandle chapter of SCBWI)
Women's Fiction Writers Association
Writer's Knowledge Base

A Great Round Up of Writing-Related Sites from OEDb

Agents and Querying Resources

Agent Query (great database of agents)
Query Tracker (another great querying resource)
Agent in the Middle (Lori Perkins)
Books and Such (Janet Kobobel, Wendy Lawton, Mary Keeley, Rachel Kent, Rachelle Gardner)
Book Ends (Jessica Faust, Kim Lionetti, and Jessica Alvarez)
Greenhouse Literary
Janet Reid
Jennifer Jackson
KT Literary (Kate Schafer Testerman, Sara Megibow, Renee Nyen)
Lucienne Diver
Pub Rants (Kristen Nelson)
Rachelle Gardner

Writers' Conference (more to come)

Here's a fantastic site with a monthly breakdown of tons of conferences from Publishing and Other Forms of Insanity

Blue Ridge Writers' Conference
SCBWI-FL Conferences
Surrey International Writers' Conference
Writer’s Digest Conference

Book Festivals (more to come)

African American Children’s Book Fair, Philadelphia, PA
Bay Area Book Festival , Berkeley, CA
Chippewa Valley Book FestivalEau Claire, WI
Faye B. Kaigler Children’s Book Festival, Hattiesburg, Mississippi
Kennesaw State University (KSU) Conference on Literature for Children and Young Adults, Kennesaw, GA
Library of Congress National Book Festival, Washington, DC
San Antonio Book Festival, San Antonio, TX
Tucson Festival of Books, Tucson, AZ
Virginia Hamilton Conference on Multicultural Literature for Youth, Kent State University, Kent, Ohio

Editors and Writing Services

Dario Ciriello
Maria D'Marco
Ava Jae
Anita Mumm
Jericho Writers Editorial Service
The History Quill
Think Story 

KM Weiland had a great list as well.