Recently, I have been looking at Pixie, to see how it works as a simple system for managing websites. It is open source (GPL v3), written to web standards (XHTML Strict, Microformats) and powered by jQuery and PHP/MySQL.
It consists of several page types – dynamic (blog, news), static or module (e.g. contact form, events, links etc). Plugins add additional functionality to modules (like allowing comments on blog posts). Blocks allow you to add content alongside your content (e.g. display RSS content from BBC News).