Instiki is a easy to use wiki that runs on all platforms (written in ruby on rails, nice and clean.)

Instiki is a basic Wiki clone so pretty and easy to set up, you’ll wonder if it’s really a wiki. Runs on Rails and focuses on portability and stability. Supports file uploads, PDF export, RSS, multiple users and password protection. Some use Instiki as a CMS - Content Management System because of its ability to export static pages.