Ghostnote is described as 'Adds contextual notes to almost anything on your mac. Add notes & todos to folders, applications, open documents, even websites. Think about it as post-it-notes for your OS' and is a Note-taking tool in the office & productivity category. There are more than 100 alternatives to Ghostnote for a variety of platforms, including Windows, Mac, Web-based, Linux and iPhone apps. The best Ghostnote alternative is Raindrop.io, which is free. Other great apps like Ghostnote are Notesnook, Zim, TagSpaces and CherryTree.