Pencil Wiki

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 :
* MembersMembers by Justin Tadlock, to manage roles and capabilities
* Breadcrumb NavXT by John Havlik, to generate breadcrumbs
* Simple Footnotes 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).