Wordpress.org
Wordpress is a very impressive blogging platform written in PHP and JavaScript. Impressive in terms of features, ease of use and stability. Wordpress comes with two default templates and two examples plugins, but you can very easily create your own templates and plugins with the Wordpress API.
Looking at the code behind Wordpress there are several classes that it uses to accomplish certain things. For example, Wordpress uses the IXR class from Incutio as a XML-RPC server/client. Doing this saves the Wordpress developers from re-writing things that other people have written so that they can concentrate on the important things.