Writing tools sit on a spectrum between proofreading and ghostwriting, and the useful ones are at the proofreading end. A grammar checker that catches a real mistake is worth paying for; a tool that rewrites your paragraph into generic corporate prose is not, and will make your writing harder to distinguish from everyone else using the same tool. The listings here separate tools that improve your own sentences from tools that replace them. If you publish anything professionally, the two habits worth building are a plain-language pass and a read-aloud pass, and most of these tools automate one of the two. Where a tool has a strong free tier, that is noted, because for most people the free tier is genuinely enough and the paid upgrade mostly buys team features you do not need.
Grammarly
Proofreading that catches more than a spellchecker.
Hemingway Editor
Highlights the sentences that are hard to read.
QuillBot
Paraphrasing tool for rewording, with a useful grammar check.
Jasper
AI writing built around brand voice and marketing workflows.
DeepL
Translation that reads like it was written in the target language.
LanguageTool
Open-source grammar checking, including self-hosted.