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 an app in the Office & Productivity category. There are more than 100 alternatives to Ghostnote for a variety of platforms, including Windows, Mac, Online / Web-based, Android and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like Ghostnote are Microsoft OneNote (Freemium), Evernote (Freemium), Simplenote (Free, Open Source) and CherryTree (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.