IkiWiki the thinking man's wiki.
More info about IkiWiki can be found at the IkiWiki home page.
I am using it simply because it can use Git as a backend and so I can develope the site remotely then upload in a controlled manner. The system then creates a nice static version of the site that does not require php or some such to be viewed and so puts very little demand on my webserver.
To get a backport for the stable version of Debian (sarge) you may need to visit GPA's site.