In the LAMP (Linux, Apache, MySql and PHP) hosting arena where most websites begin life, there are several popular CMS's (Content management Systems) to choose from. In our opinion these are Wordpress, Drupal, Joomla and the upcoming Concrete5. Each has their strengths and weaknesses and the decision on which one to use will be based on several factors such as the user's intention and ability to edit content and what extra features are required. For e-commerce Magento is a strong product or there are commerce modules for the previously mentioned CMS's.
We have experience will all these CMS's as well as PHP MVC frameworks that are not specifically CMS's. When performance is crucial we use the Yii framework.
Incidently PHP is the most used server-side language on the Web, powering 77% of all database-driven websites including Facebook and Wikipedia.