UV Outliner - UV Outliner is a small and powerful single-pane outliner.
Documize - Enterprise-grade wiki and knowledge management platform
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
Planka - Planka is an open source project management software similar to Trello.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.