DownMarker is described as 'viewer and editor for markdown files'. There are more than 50 alternatives to DownMarker for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Android. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Typora or Mark Text. Other great apps like DownMarker are ghostwriter (Free, Open Source), StackEdit (Free, Open Source), Inkdrop (Paid) and ReText (Free, Open Source).
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, can be copied, tagged and modified either from the applications directly or from your own text editor.
No excuses. No distractions. Just write. ghostwriter is a feature-rich text editor using Markdown syntax which makes it suitable for note-taking, document redaction, simple HTML website creation and much more.
ReText is a simple text editor for Markdown and reStructuredText documents with syntax highlighting and optional live preview. It is written in Python using Qt libraries, able to run on any platforms (Linux and BSD are officially supported).