While great tools won’t make you a great writer, they can help you grow and learn if used right. Here’s what I use. (Oh, and a disclaimer: none of the links are affiliated. I’m not trying to sell you anything.)
Google Ngram Viewer – Searches Google’s database of books to see how often certain phrases repeat. Really good when you have two similar expressions fluttering around your head, and a strong suspicion one of them isn’t right. (like red on the face vs red in the face)
Submission Grinder – A free tool for tracking your manuscript submissions. A paid alternative is called Duotrope.
Grammarly – In my opinion, the smartest spell and style checker out there. It even detects things like when you should’ve used they’re and their. Mind you, it’s not a real human being so it may miss a context of your sentence occasionally, but if used consciously can really clean up that manuscript of yours.
AutoCrit – Pretty expensive in my opinion, but worth giving a trial. It will give you lots of pointers on things writers tend to skim over, like passive voice and showing vs telling. It will mercilessly point out all adverbs in your dialogue, and do a dozen other cool things besides.
Thesaurus.com – a comprehensive thesaurus that helped me grow my vocabulary. I really liked how it graded the suggestions moving further away from the searched phrase. Also gives you antonyms and related phrases.
Dictionary.com – a sister page to thesaurus.com. Not much else to say. A dictionary. Has words in it.
Roget.org – an online version of the famous Roget’s Thesaurus. Synonyms and their meanings are organized into contexts so if you search for a word like cool, you will get a separate list of synonyms for temperature, and temperament.
Power Thesaurus – haven’t used this one much, but I keep it in my toolbar anyway because it gives you synonyms ranked by popularity.
Describing Words – a fantastic resource for any writer provides you with a list of adjectives commonly used to describe a noun. Ever wonder how you could describe a nose, a door, a carpet? Look no further!
Urban Thesaurus – for searching slang.
Behind the Name – names from all cultures, and most importantly, with their meaning. Great when you’re looking for a name for one of your characters.
Ulysses App – This one is macOS only. In principle, I avoid platform-specific apps, but this one won my heart with a simple interface, markdown editor, and a system of inline comments you can add and edit without lifting your fingers off the keyboard.
Evernote – Hardly needs an introduction. All your chaotic notes stored in one place. I employ an elaborate system of tags to find my way around thousands of my notes, references, and ideas.
Scrivener – I used it for years. The interface is too bloated for my taste, but the app comes with some really useful features helping you plan and organize your novel or a screenplay. You can configure it to save your writing, whatever its original format, into the Standard Manuscript Format.
The Ultimate Tool
Pen and Paper – The last tool on the list is my favorite. I always keep a blank sheet of paper around.