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, which is both free and Open Source. Other great apps like Ghostnote are Standard Notes (Freemium, Open Source), Google Keep (Free), Microsoft OneNote (Freemium) and Evernote (Freemium).
Notebook PEAs - encryption of notes. Protects your private notes with a password. The text is never stored unencrypted on your disk (until you explicitly decide to do that). The used cryptographic primitives are currently considered as safe as possible.
PNotes (Pinned Notes or Portable Notes, use what you prefer) is written entirely in C and Windows API (with Pelles C for Windows IDE) - so it is fast and light-weight. It has flexible settings - both for the program in whole and for individual note.
NoteAway is the simplest, most convenient way to take quick notes on OS X. NoteAway has lots of powerful features for free, like iCloud syncing, rich text, attachments, simple import/export, and a variety of customizable options.