Outline

Outline is an open-source, team knowledge base built with a focus on seamless integration with Slack. It offers a clean, Markdown-based editor, tree-based page hierarchy, and collaborative editing. It's designed for companies that want a modern, self-hostable wiki that encourages adoption through Slack notifications and search. It's free to self-host.

It ranks for its excellent Slack integration and clean design, making it a strong contender for tech-savvy teams prioritizing internal collaboration.