I maintain a number of sites that use PmWiki [1] as a CMS. The community's small (though active), it's in PHP, and the editing isn't WYSIWYG, but the reason I ended up with it in the first place is how well the framework is organised and how easily add-ons can be packaged and included to add to or even replace core functionality. Templates are also completely separate from the PHP.
One feature of PmWiki that I've yet to come across in any other CMS are PageLists, which allow you fetch semi-structured data and other content from the rest of the site and displaying it using a custom template.
One feature of PmWiki that I've yet to come across in any other CMS are PageLists, which allow you fetch semi-structured data and other content from the rest of the site and displaying it using a custom template.
[1] http://www.pmwiki.org/