See https://github.com/cjbarber/hipchat-alternatives for more ideas.
Besides a gazillion open source wiki projects…
For a bigger far more all encompassing documentation toolchain:
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Jive Disclosure: former employer (but thoroughly recommend for the right culture/environment)
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Salesforce.com’s Chatter Disclosure: former employer (don’t recommend based on user experience 3+ years ago)
We recently switched from Confluence to AllAnswered and really loved it. AllAnswered combines Wiki documents like Confluence and Questions & Answers similar to Stack Overflow. Great product and very easy to use. Here is their website https://www.allanswered.com