pmWiki -- a flat-file, PHP (!!!) wiki from "the pumpking for the Rakudo Perl 6 compiler." Okay, so he's no longer the prime, active maintainer, but he's got chops! And the work he put into pmWiki made it a flexible, extensible, secure multi-user simultaneously-editable CMS.
I use pmwiki for a personal site, a commercial-oriented-site and as a departmental knowledge-base-behind-a-work-firewall in my department at work.
It's easy to set up, easy to extend, well-documented, and has a great dev community with ready responses to issues.
Plus, it's a wiki, dammit. Wikis rock! I can create pages with nothing but link-dumps that I can can then organize, re-org, cross-reference, add notes, screenshots, code-samples, and revisit months and years later. It's not in the cloud, so what. I own the data. It's mine!