A friend and fellow traveler as an independent author asked what three writing tools I couldn't do without, so here's my list. I'll focus on tools for fiction writing for now, because the list varies when we're talking about writing plays, screenplays, or technical documents.
For capturing and sketching out ideas, I start with Evernote, because it's quick, easy, and accessible from every word-writing device I own (smartphone, tablet, laptop, desktop, public computer at the hotel, etc.). My mind is like a little magpie, always distracted by the shiny new scientific discovery, fictional civilization idea, snippet of dialogue, character note, cool name, or other thing I should remember. I don't lose them with Evernote. I could wish it did a few extra things, like allow sub-notebooks, or more formatting for the web version, or easier download/backups of the Web version for we-who-are-paranoid, but those complaints are minor. With the Evernote client application version, you can even work offline and sync to the master content on the web once you get reconnected to the internet.
For writing, I'm in a crossover between MS Word and Scrivener. I have a lot of experience using Word, I know its strengths and weaknesses, and it's a ubiquitous format that editors, beta readers, ebook file converters, etc. can handle. That said, its weaknesses are increasingly annoying. If you write science fiction like I do, with a lot of new vocabulary, Word gives up displaying spelling errors once your manuscript hits about 40,000 words. If you aren't careful with styles, the process to export the file for ebook conversion is unpredictable at best. Word is designed on the assumption that you'll be printing on 8½ x 11-inch paper, which is not really the case when you're writing fiction. Yes, you can adjust page size, but it still assumes you'll be printing on your home printer.
I'm new to Scrivener and I like what I see so far, and plan to use it for my upcoming novella to get to know the software better. It cleanly exports or publishes to a variety of formats, including Word, PDF, .ePub, and .MOBI, or so I'm told. Mostly I like the uncluttered interface and the ability to keep front matter, research, etc. in separate files. I haven't figured out the rest of its features, some of which I'm sure could help my process. I'm also told it has a steep learning curve to get it to sing and dance, so I'll have to see if it ultimately helps or leaves me flummoxed. BTW, all the copy editors I know want the full MSS in Word when you're ready to send it to them, and you'll need the Word file for conversion to the e-book formats, too.
I live and die by the internet (or the World Wide Web, for technical purists). I am completely and thoroughly spoiled by having the world of references at my fingertips. If someone hasn't already written a fabulous article on the very subject you're researching (e.g., how many minutes a human could live in space without a spacesuit – https://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html ), or written an answer to someone else's similar question, you can track down forums or blogs where other readers know the answers. I found someone who is bilingual in English and Icelandic that way, which was exactly what I needed for my upcoming novel, Overload Flux. I donate money every year to Wikipedia because they're a quick (though not definitive) overview of an amazing number of subjects, such as theoretical quantum physics, volcanism, the history of ocean navigation, genetic modification, the parts of a rifle, etc. Even if Wikipedia isn't always trustworthy because pretty much anyone can edit a page, it can at least point you to more authoritative sources.
If you have tools you couldn't live without for writing, feel free to recommend them in the comments.
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P.S. I'm not a reseller or affiliate of either Evernote or Scrivner, just a happy customer.