Google Docs
A powerful document creation platform. Supports features such as: realtime collaboration, offline document editing, inline document comments, search and full differential changelog. Free for personal use, costs for commercial
documentation free commercial
Hackpad
A lightweight, markdown based, document creation platform packed full of features such as: realtime collaboration, offline document reading (via dropbox), in documment comments, search / tagging and full diff'd changelog. Private workspaces free for the first 5 users, Public workspaces always free.
documentation free commercial