Qwicky is a REALLY small wiki written in Ruby using Sinatra, DataMapper and SQLite3.
It's features are:
* It has pages
* You can edit them
* You can delete them
* You can rename them
* You can set a homepage
* You can use markup (RDoc, Textile, Markdown or plain text)
* It has a simple site-map to give you an overview of all the pages
* It knows wiki links
Setting it up takes about 2 minutes, including download time ;-)...
It's easily run on servers.