I've found Instiki to be the easiest wiki wiki to setup - especially on OSX. It's written in Ruby, so you'll have to install Ruby first if you haven't already (on OSX the install package comes with the latest version of Ruby, so there are no worries on OSX).
Instiki also allows you to export pdf, TeX or http.
Here's a list of features:Included
* No dependencies: Ships with an embedded webserver (WEBrick)
* Regular expression search: Find deep stuff really fast
* Revisions: Follow the changes on every page from birth or rollback a revision
* Export to HTML or markup in a zip: Take the entire wiki home with you
* RSS feeds: Track recently revised pages by headlines or content
* Multiple webs: Create separate wikis with their own namespace
* Password-protected webs
* Authors: Each revision is associated with an authorwho wrote what?
* Reference tracker: Which other pages are pointing to the current?
* Speed: Uses Madeleine for persistence (all pages are in memory)
* Three choices for markup: Textile (Red Cloth), Markdown (Blue Cloth), RDoc
* Internationalization: Write Camel Case Wiki Words in all Latin, Greek, Cyrillic, or Armenian characters or Bracket Style Wiki Words in any language.
* Specialization: Add rules to the stylesheet to change the look entirely or just pick one of the predefined colors.
* Keyboard Shortcuts: ALT+E for edit page, etc.