Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
Joplin - 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, tagged and modified either from the applications directly or from your own text editor.
GitJournal - Manage your Notes from any Git Repo.
Standard Notes - A safe place for your notes, thoughts, and life's work
Quillnote - Take beautiful markdown notes and stay organized with task lists.
Tomboy - Apps/Tomboy - GNOME Wiki!