Google Docs, Rentry.co, Nuclino, Etherpad, Mark Text, Firepad, PiratePad, HedgeDoc, Deckset, StackEdit, Rustpad, Twake, Zoho Writer, Documize, Framapad, hackpad, Slidium, Madoko, Elegant Markdown Editor, Slideas, GitPitch, Etherpad.net are some alternatives to HackMD.