Silverstripe is built on the Sapphire framework, which is a powerful foundation for you to build from. It boasts an easy-to-use object-relational mapper to create relationships between web pages and data objects. It also has one of the nicest site map trees on the market today, making it intuitive to use and understand. It has a decent user community, and you certainly can’t beat the price (free).
A very flexible, modular platform for a powerful out of the box CMS.