Buho is a note-taking application that lets you write the computer equivalent of sticky notes.
Linwood Butterfly
Powerful, minimalistic, cross-platform, opensource note-taking app.
Writernote
Multiplatform application, currently available for linux, windows, macOS and android, which allows you to take notes in an intelligent way, you can record audio as you write, and listen to it again seeing what you have written for every second of the audio.
SimpleNote
The simplest way to keep notes
GTG
(GTG) is a personal tasks and TODO-list items organizer for the GNOME desktop environment
FeatherNotes
Lightweight Qt5 Notes-Manager for Linux