If you are a web developer or just a hobby hacker take a look at this video. It shows how to create a Wordpress plug-in so that you can extend Wordpress to be more than just a blog engine. An understanding of PHP is required of course.
The most impressive thing is the fact that the expalination and working code are done in the short space of a Youtube video. I don't think any other CMS project will be able to make the same claim. Wordpress is number one for a reason.
Happy Publishing!