Why have I never heard of Pivot before? it’s a gorgeous GPL-licensed PHP-powered blogging tool. I wouldn’t drop WordPress for it unless there was a way to convert my entries and preserve the hyperlinks — I’ve disrupted my blog universe too much in the past year. But I will definitely look at it for future blogging projects.
UPDATE: downside to Pivot: does not yet use Blogger or Atom XML-RPC api’s, so you can’t use remote blogging tools like MarsEdit or Ecto to post to it.
It has excellent “multiple blogs/multiple users” capabilities though. Neat stuff. Oh, and it has an *incredible* image upload/thumbnailing feature.