Qwicky is a REALLY small wiki written in Ruby using Sinatra, DataMapper and SQLite3.

• 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.