Apps with 'Real-time inline preview' feature
a markdown editor, markdown reader.
Mark Text is a Markdown editor for Mac, Windows and Linux. It is a concise text editor, dedicated to improving your editing efficiency.
Qilin is a free, open-source and fully hackable text editor developed for exact sciences in mind. It fully supports KaTeX and AsciiMath. Qilin works on each desktop platform - it is based on Chromium and Node.js, licensed under the MIT license.