Rustpad - Open-source collaborative text editor based on the operational transformation algorithm.
Documize - Enterprise-grade wiki and knowledge management platform
Plunker - Plunker is an online community for creating, collaborating on and sharing your web development ideas.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
collabedit - Collabedit is an online and collaborative text editing program suited to programmers and ideal for job interviews, educational lessons, and team projects.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.