Google Docs is a popular collaborative writing app that offers robust collaboration features, including real-time commenting, version history, and exporting options. It's free and integrates well with other Google apps. Microsoft Word Online is another option that excels in compatibility with other Microsoft Office files and quick document formatting. Dropbox Paper offers a clean, clutter-free design and rich media support, while Etherpad provides a plain-text-like interface and open-source features for developers. Quip combines text documents, spreadsheets, and checklists in a central place, making it ideal for teams. Hackpad is a living document or online notebook for co-editing notes, with document moderation and organization tools. Other collaborative writing apps include Zoho Writer, Penflip, Notion.so, and GatherContent, each with their own strengths and weaknesses.