I started my first wiki today. I'd point to it, but it's behind the firewall. It's using PMWiki, which was chosen through a thorough process of searching for "php wiki" (everyone has a www directory in their home directory that supports php) and clicking on a few links. Total up-and-editing-time was about seven minutes. Neat. I made about 32 pages, which was also fun (most of them short and pasted).
I'm undecided on it in general. I've always that the idea of a wiki behind the firewall was more appealing than not. It'll certainly be interesting to learn from.
Addendum: Just saw the Times piece from yesterday: "Lately, though, the business world has begun experimenting with wikis." Oh, good. Nice mention for SocialText.