While engaging in continuing education and research, I have come across these informative websites. I found them helpful and/or promising for future exploration. I hope you do too.
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(Note: I have no affiliation with these sites or businesses.)
Andrea (Andi) Cumbo is a writer, developmental editor, speaker, workshop facilitator, and writing coach.  She has also developed a writer's support community.
Conscious Style Guide is an amazing resource subtitled: include/empower/respect! It has links to wonderful articles describing the strides the publishing industry is making (and the need for more) toward more inclusivity. I have a hard time tearing myself away from this site.
The Daily Writing Tips are cool. I've been a subscriber for years and can always learn something new.
The Editor's Blog is a great resource for writers as well as editors. Editor and writer Beth Hill offers practical advice and lots of great resource links to other good sites, webinars, and E-zines. No Comma Necessary—Coordinating Conjunctions Don't Always Need Commas has some of the best advice I've found (in the Comments as well) on the nuances of comma usage!
Emma Darwin's: This Itch of Writing is a FANTASTIC resource for writers! Emma Darwin is an amazing historical fiction writer and blogger on fiction and creative non-fiction issues.
Grammar Book is a good reference based on The Blue Book of Grammar and Punctuation by Jane Straus. It's beautifully and simply organized for searching out specifics on ... grammar and punctuation rules! (Yes! You guessed that one, huh? You are amazing!) It's a great tool and resource and good to keep on speed dial, as it were (I'm dating myself) or in an open tab on your laptop. :)
Grammar Girl (aka Mignon Fogarty)—blogger, linguist, and author—has a popular podcast and a website —Quick and Dirty Tips. She's my go-to gal, and I've got a number of her books. She's brilliant and easy to understand! 
Helping Writers Become Authors is the goal of K.M. (Katie) Weiland—blogger, mentor, educator, and author of fiction books as well as resources for writers. Her website/blog is chock full of amazing resources for learning and honing the craft of writing, e.g., story and scene structure, outlining, character development, etc.
Check out the free website service where you can copy your own text and paste/replace the example text on the landing page to get an evaluation. Hemmingwayapp.com. Note: the desktop app for working offline is not free, but the online version is.
The Independent Book Publishers Association has a free pdf of the Industry Standards Checklist for a professionally published book.
Jami Gold is a writer, editor, workshop host, blogger, and entrepreneur. Jami has some wonderful Worksheets and Workshops as well as articles on writing.
Jericho Writers - resources for writers like tutorials, webinars, chat groups, and agents (you need membership for full access, but blog posts are free.) I found an article on character development to share with my authors, and one on writing dialogue, and another on developing your authorial voice. Plus one about Evoking a Sense of Place early in a scene! 
Kristen Lamb is an author, blogger, speaker, and social media Jedi. She tackles many topics including Why Editing Matters and Simple Ways to Make Your Work SHINE.
Laurel Garver, author and editor, has a blog (Laurel's Leaves) with a fantastic explanation for why authors need critique partners and beta readers and how to get the most out of both.
Lisa Poisso, editor and book coach, has written a lovely article explaining why perfection is not possible: Why Did the Editor Miss Errors in Your Book? Among other excellent articles is: The Writer's Guide to Hiring an Editor.
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Literary Devices  is a website that not only lists and explains literary devices and terms, and grammatical terms, but also offers poem analysis, a very comprehensive list and analysis of common phrases (bated breath, the lady doth protest too much, silence is golden, etc.), and essay writing descriptions. One of my favorite articles is the discussion of literary perspectives.
Louise Harnby is an editor, proofreader, and mentor to many an editor. She has incredible resources! 
Megan Harris is an editor, blogger, and copywriter. She has great articles and wisdom to share regarding contracts between editors and writers and about appropriate ways of offering thanks to your editor for a job well-done.
NY Book Editors website has many helpful articles in the blog section. One sweet article is about getting your manuscript ready to submit to an editor :) Don't Submit Your Manuscript Until You've Read This Post. Also: How to Plan a Book Series and: What's the Difference Between Perspective and Point of View, and: What You Need to Know About Working with Beta Readers.
The Online Etymology Dictionary, or Etymonline, is so cool. If you are writing historical fiction, one pitfall you need to avoid is using anachronisms: history, events, words, or idioms that didn't exist in that era.
The Punctuation Guide  is a phenomenal tool/reference. It's easy to use and great for refreshing your memory about how and when to use semicolons, em dashes, quotation marks, etc. It also has information on Manuals of Style, Top Ten Tips (and we all love those! Admit it!), British vs American English, and more.
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reedsy blog  is a clearing house for all sorts of self-publishing tools and assistance. They have a terrific list of writing community/critique groups: 50 Places to find a critique circle to improve your writing and free creative writing exercises.
Steven Pressfield is a novelist as well as the author of The War of Art and other books about writing. His website offers great insights and philosophies, tutorials, and practical advice on how to make a career out of writing.
Storm Writing School - Tim Storm is an educator, a writing craft coach, and an editor. He creates wonderful articles, events, and courses, and he has great YouTube videos as well. I especially love this article on Readers who help you through the "curse of knowledge," your own biases, and perspectives.
The Story Grid was created by editor Shawn Coyne. Wow. Pretty cool stuff. He has interesting and helpful videos, books, and blog posts.
Frank McKinley's Thriving Writers website has an excellent article called Why Every Writer Needs an Editor by guest blogger Emily Clark. Frank also has a Thriving Writers show on YouTube that looks informative.
The Write Life has articles on a plethora of topics by different authors, bloggers, editors, etc. Check out: The Worst Ways to Begin Your Novel or: How to Format a Book—10 Tips Your Editor Wants You to Know. :)
The Write Practice has a blog post I especially appreciated about Point of View. The website has helpful guidance for practicing in order to progress, and a supportive community for aspiring writers.
Writer's Digest blog has amazing articles. I found a helpful article on Copyright Laws for writers. And check out the article on weaving backstory into your novel. It has a huge selection of free stuff, tutorials, webinars, articles, prompts, etc. on genre-specific topics, publishing issues, writing techniques, etc.
Writers Write has some great articles like How to Write an Ethically and Socially Sensitive Story, advice like The Edit, writing courses they offer, and cool literary stuff! You'll be going back for more!
Word-of-the-Day mailings from Merriam-Webster's FREE online dictionary—way better than one of those tear-off calendars! The newsletter links to quizzes to test your word power and other cool things. It's fun if you are competitive and geeky about vocabulary. I also consult the website daily for definitions, synonyms, etymologies, hyphenation checks, etc. I recently found this great explanation of the subjunctive mood (e.g. "If I were a rich man, ...").
Have you found a website, blog, or other resource to be especially helpful?
Please share it with me in the Contact Form. Thanks!