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.

Boone Gorges

CUNY Academic Commons

David Cavins

BuddyPress Docs adds collaborative work spaces to your BuddyPress community. Part wiki, part document editing, part shared dropbox, think of these Docs as a BuddyPress version of the Docs service offered by the Big G ifyouknowwhatimean
Features include:
<ul>
<li>Docs that can be linked to groups or users, with a variety of privacy levels</li>
<li>Support for fully-private document uploads</li>
<li>Doc taxonomy, using tags</li>
<li>Fully sortable and filterable doc lists</li>
<li>TinyMCE front-end doc editing</li>
<li>One-editor-at-a-time prevention against overwrites, plus idle detection/autosave</li>
<li>Full access to revision history</li>
<li>Dashboard access and management of Docs for the site admin</li>
</ul>
This plugin is in active development. For feature requests and bug reports, visit http://github.com/boonebgorges/buddypress-docs. If you have translated the plugin and would like to provide your translation for distribution with BuddyPress Docs, please contact the plugin author.

BuddyPress Docs

I used it on a project for a client several years ago and it worked like a charm. Also, it was well-maintained.

Pencil Wiki vs BuddyPress Docs