Slack - A messaging app for teams who see through the Earth!
Documize - Enterprise-grade wiki and knowledge management platform
Mattermost - Mattermost is an open source alternative to Slack.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Tandem - Get fluent together. Practice languages, anytime, anywhere.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.