For a writing app that uses AI to help with brainstorming, outlining and editing, Strut is a good option. Strut is a collaborative writing environment that combines notes, documents and projects with AI tools for writing tasks. It includes an AI collaborator for brainstorming, outlining and drafting, and inline editing for grammar and style. It also works offline and has a distraction-free environment for focused writing workflow.
Narrato is another all-purpose platform that packages AI tools to help you create content. It includes an AI Content Assistant with more than 100 tools, an AI Content Genie for automated content creation and content workflow automation. Narrato is geared for content marketers, writers and teams that need to create high-quality content more quickly and efficiently.
If you want an AI writing assistant that's tuned to tone and grammar, Writetone is worth a look. It's got tools like Paraphraser, Co-Writer for brainstorming and editing, and a Grammar Check for proofreading. Writetone is available as a Chrome extension and through MS Word integration, so it's a good option for writers, marketers and students.