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.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Boostnote - Boostnote is an open-source note-taking app.
txt2tags - It reads a text file with minimal markup such as **bold** and //italic// and converts it to the...
Standard Notes - A safe place for your notes, thoughts, and life's work
Texy! - Texy! is a lightweight markup language as well as converter of this format to XHTML, in a form of a...