Wikis are wonderful things! So simple, but so powerful. At their core, simply a collection of named text documents that follow some reasonably simple syntax rules. Make these editable over the web, and searchable, and pretty... and there's a wonderful website that doesn't try to tell its editors how to structure their work, and just lets them write. If you want date-based articles, go for it; or a page for each photograph, that's easy too; some weird industry-specific nomenclature? Anything goes really!

I work for the WMF, and so of course I use MediaWiki. I've used other wiki engines though — for years I loved PmWiki, and after that Dokuwiki — and they're all pretty good. The fundamental tenets remain the same.

The Universal Edit Button is a great addition to any wiki, it lets you get into editing mode from anywhere on the page at any time by clicking an icon in the address bar (doesn't work for mobile browsers it seems).