Soks is yet another Ruby wiki. See http://www.soks.org/view/Home Page (the link seems to be dead).
* Automatic linking between pages (no need for wiki words or any bracket shenanigans).
* Easy to add, move and delete pages
* Calendars, automatic summaries and the like
* An api for classes to modify the wiki
* Flat file storage (incidentally, this makes it easy to import from instiki using its export function)
* Easy to edit the look of the wiki with per wiki templates.
* Variety of ways of authenticating, with the authentication scheme selected based on a regexp of the page title.
* Built in web server (No need for Apache)
* Textile
* Uploads and Images
* RSS feed for changes to any page