Outline is a open-source knowledge base with beautiful design, real-time collaboration, and API. It handles open source, real-time, api, and markdown, and it's best suited for teams wanting a fast, beautiful open-source wiki. It has been around since 2017, giving it a 9-year head start in building out integrations and refining the product.

The core product is entirely free. Since it's open-source, you can self-host for free with no user limits. It's designed for small teams, striking a balance between simplicity and collaboration features.