Interesting article on the dangers of not keeping software patched, particularly with regards to open source content management systems.
gelato cms is described as a tumblelog cms. It's based around the tumbr concept, you can easily post snippets. Various themes and plugins are available.
Wiki written in python as used by Ubuntu, FreeBSD, Debian and others. Uses flat file storage by default. Lovely.
Radiant CMS is a simple CMS system built in Ruby on Rails. It has a plugin interface so it can be extended.
Create a web site and a blog on your own domain, for free. Powers Jessica Livingston's Founders at Work site.