Ask HN: What's the best internal knowledge base

Our company is scaling, and we're looking for a knowledge base to use internally to help train new hires and share knowledge across departments.

We're looking for good security and stability, versioning of some kind so changes aren't lost, ease-of-use so computer novices can comfortably edit things, and solid full-text search.

In a perfect world, we'd find one that's open source that we can host ourselves.

Does anyone have any recommendations?