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.
A5 - Takes notes in your browser. Elegant & simple.
Boostnote - Boostnote is an open-source note-taking app.
Falcon for iOS - Writing notes made simple
Typora - A minimal Markdown reading & writing app.
Chars - Unicode Symbols Keyboard for iOS