StackEdit is described as 'free, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.Rich Markdown editor' and is a popular app in the Development category. There are more than 50 alternatives to StackEdit for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Android. The best alternative is Joplin, which is both free and Open Source. Other great apps like StackEdit are Typora (Free), Zettlr (Free, Open Source), Mark Text (Free, Open Source) and Trilium Notes (Free, Open Source).
Keenwrite is a Java-based desktop text editor with live preview, string interpolation, and formulas. With real-time spell checking, Tex-based match rendering, document statistics and even https://alternativeto.