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With all it's warts, confluence is pretty good. It's not good software, it runs like ass, has a crappy markdown editor. But it supports attachments, has a great jira integration, solid-ish markdown rendering, hierarchal pages, multiple projects, separated viewing and editing paradigms. But it's still confluence.

I've seen good use with Slab

Confluence, the product is very far from perfect, but still does the job well. I'm working on building the perfect knowledge base tool, but that would be for big companies

A WordPress website with Jetpack search is surprisingly good — we use something like that at Automattic. All long-lived documentation goes in the “fieldguide” website, and it’s open (and encouraged) for anyone to add or edit pages.