grosbouff

Pencil Wiki is a simple wiki solution for your WordPress.
Basically, it adds new roles & capabilities that allow your users to add/edit wiki pages (custom post type).
It comes with several widgets and custom templates (that you can override) fully integrated with the Twenty Twelve theme.
Pages are edited backend, but there is a menu on every wiki page with the necessary links; and a wiki index / search input.
Only Administrators and Editors can create new wikis (top-level pages); and Wiki Authors can add pages to it. 
Administrators and Editors can also LOCK some wiki pages (for any reason) or branches; which means they will not be editable anymore by other users.
I voluntarily made the plugin quite simple to avoid making it intrusive (javascript, css, …) and put too many features. I recommend to run it with those plugins : 
* <a href="https://wordpress.org/extend/plugins/member" rel="ugc">Members</a>Members by Justin Tadlock, to manage roles and capabilities 
* <a href="http://mtekk.us/code/breadcrumb-navxt" rel="nofollow ugc">Breadcrumb NavXT</a> by John Havlik, to generate breadcrumbs 
* <a href="https://wordpress.org/extend/plugins/simple-footnotes" rel="ugc">Simple Footnotes</a> by Andrew Nacin, to put footnotes in your wiki pages.
WordPress has a revision system, which is perfect for a wiki. 
Unfortunately, I didn’t find a way to customize it (add revision messages, etc).

Pencil Wiki

Pencil Wiki is a simple wiki solution for your Wordpress.

WP HelpKit

Create, Organize and Publish help content.
A simple tool for creating Knowledge Base and Documentation sites 
for your software products.
HelpKit for documentation sites 
is what WooCommerce is for eCommerce sites.
<h3>What can I do with HelpKit?</h3>
<ul>
<li>Help Center / Support Sites</li>
<li>Knowledge Base</li>
<li>Documentation</li>
<li>User Manuals</li>
<li>Wiki</li>
<li>FAQs</li>
</ul>

WPHelpKit

We've just started using it to document our WooCommerce themes and it seems to be exactly what we were looking for.

Exactly what we need

Create a fully featured Help Center site (Knowledge Base, Documentation, Wiki, FAQs) with WordPress.

Pencil Wiki vs WPHelpKit