PyroCMS is a fantastic CMS built upon the CodeIgniter framework. Out of the box it provides all the features you would need to build a website and it can be extended, improved and added upon through its add-on system.
It is simmiler to WordPress but unlike WordPress, PyroCMS has a much nicer codebase so you don't feal dirty working with it!

This website is built using PyroCMS and it is also the system I use for and recomend to my clients.

As I have mentioned PyroCMS can be extended using add-ons, these can be themes for the front and back end, modules which provide a whole lump of functionality such as a gallery and widgets and plugins which make adding things like share buttons or maps really easy.
I have built a number of add-ons for my websites and my clients, some of these will hopefully be usefull for others so I am listing them in the add-on store.