research : wikis

Maiahost.com

free install of Mediawiki, Docuwiki, Tikiwiki, Wordpress, MovableType

helendewitt.wetpaint.com

Good: collapsible, hierarchical sidebar menu
Bad: Horrible obtrusive ads across top

helendewitt.wikispaces.com

Goodish: clean, simple theme, automatic tabs for page, discussion etc.
Bad: Can't do parent-child pages, can only set up collapsible sidebar menu with a lot of faffing around in code

DeWiki (this site) - wikidot

best so far (but after four days wondering whether I should start from scratch elsewhere, hence checking out maiahost.com with free installation of docuwiki or one of a few others)

Good: parent-child hierarchy easy to set up, decent selection of themes, ads so unobtrusive I've never noticed them

Bad: Hours and hours of searching documentation and forums have not turned up a way to provide the most intuitive navigation: collapsible list of pages in a category with MY order of pages preserved in each list and sublist - something like the foldable TOC, only with the items links to other pages rather than to headings on the same page. Quite possibly achievable with CSS, hence excursion into couple of hours of looking for a CSS IDE (point at which I wondered whether I should start again from scratch)

Bad: documentation not always sufficiently fleshed out (one cd argue, at this point, that you get what you pay for, except that there's no reason to think documentation is better if you pay for an upgrade)