Doing Content Right - The go-to resource for creating content online
Documize - Enterprise-grade wiki and knowledge management platform
Collato - Collato: A smart notetaker for busy people. Website: https://collato.com/
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Monogram Creative Console - Creative Console is a modular, freeform control surface designed to streamline editing and make interacting with creative software more engaging.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.