You Need A Wiki - Create a wiki with Google Docs
ProProfs Knowledge Base - ProProfs Knowledge Base is a cutting-edge knowledge management software that assists businesses to reduces support tickets by self-help and create online manuals, guides, wikis, documentation, FAQs and more.
Stack Overflow Documentation - A crowdsourced developer documentation
Docular - Docular keeps your knowledge base healthy by automatically sending notifications to relevant team members about articles that need to be kept up to date and relevant.
Greenboard - Generate beautiful static api documentation
Kbee - Create a wiki in 5 minutes Kbee turns your Google Drive folders into a fast, searchable wiki for you and your team.