We are looking for a lightweight knowledge base system, with roughly these requirements:
Do you use such a system, or develop one? It’s time to reply on this post 😉
It may sound terrible, but why not WordPress? See athemes.com/collections/best-knowledge-base-wiki-…
You can actually get Confluence Server for $10/yr (up to 10 users, with renewals being $10/yr, if you care about updates - if not, it'll still run on the last version you've updated it to), and - I have to say - a pretty badass VPS for $7.00/mo @ VPSDime.
Maybe just MediaWiki (like Wikipedia)?
Confluence is popular of course, but pretty expensive, and it doesn't sound like you need whatever extras it offers over free alternatives.
Lorefnon
Open Web Enthusiast
I have been using gollum for quite some time. If you don't mind a ruby based application, it fits all the criteria.