Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
SnipNotes - SnipNotes is your clever notebook for iPhone, iPad, Mac and Apple Watch.
MkDocs - Project documentation with Markdown.
Trilium Notes - Trilium Notes is a hierarchical note taking application.
Docusaurus - Easy to maintain open source documentation websites