Oddmuse http://www.oddmuse.org/ is a good candidate. It's Perl+flat file, reasonably simple to use and maintain.
Depending on your needs, you might also consider TiddlyWiki http://www.tiddlywiki.com/, which is implemented entirely in client-side JavaScript, stored in a single HTML file which includes the script and the data. There are many add-ons and customizations for it, and there are ways to save it to the local filesystem and/or a "ServerSide".
Probably unsuitable, but including it for completeness' sake: Sig Wik http://c2.com/cgi/wiki?SigWik, which is implemented in 222 characters of Perl.
Pete
Dawson wrote:
Can anyone recommend a really lightweight Wiki? Preferably PHP but flat file would be considered too.