HackMD is described as 'browser-based, real-time collaborative markdown editor. It was inspired by hackpad but focuses more on speed and flexibility' and is an app in the Office & Productivity category. There are more than 25 alternatives to HackMD for a variety of platforms, including the Web, Mac, Linux, Windows and Self-Hosted solutions. The best alternative is Google Docs, which is free. Other great apps like HackMD are Mark Text (Free, Open Source), StackEdit (Free, Open Source), Etherpad (Free, Open Source) and Nuclino (Freemium).