I've been using jumpbox list of wikis to evaluate some. Here are my initialconclusions (based on comparing the writeup with my requirements):
I particularly like two points from their philosophy:
1. Favor writers over readerstikiwiki
3. Avoid gratuitous features (or "creeping featurism")
TikiWiki is your Groupware/Content Management System solution with a long list of features to help you build a compelling web based community: Wikis, Forums, Blogs, Articles, Image Gallery, Map Server, Link Directory, Translation and i18n. And much more...
twikiwiki (Watch the extra w)
TWiki is a flexible, powerful, and easy to use enterprise wiki, enterprise collaboration platform and knowledge management system. It is a Structured Wiki, typically used to run a project development space, a document management system, a knowledge base, or any other groupware tool, on an intranet or on the internet.
mediawiki, as used by wikipedia
Promising. Slightly too much stuff on the page, but can be configured away.
Specifically designed for creating documentation, however you can't get email notifications when a page changes, which is a showstopper for me.
So that's three to look at. I haven't evaluated them on ease of installation, or maturity of product, or how much support there is, or how widely they are used. More to come...
Update: 17 March 2009 on using mediawiki for documentation