Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Documize - Enterprise-grade wiki and knowledge management platform
Forumbee - Forumbee offers community forums for discussions, feedback and Q&A.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
Directly - Directly provides on-demand customer service apps.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.