Drupal
TMA has worked with Drupal extensively and speaks from a solid foundation of experience.
Drupal is a content management system that has seen a lot of growth both in user base and its core development in the last few years. It is an excellent CMS and favored by many programmers and other early adopters looking to carve out a market niche. Drupal out of the box is simply a framework from which plug-ins must then be installed to really do anything. It's this open and empty framework tha appeals to many giving the administrator the flexibility to chose how best to manage the system.
While the third party development community isn't has large as some of the other CMS projects those who do develop for this platform tend to be zealous about Drupal as well as the philosophy behind it. Drupal is perhaps the one CMS that adheres most to the opens source / free ethos and is recommended to clients who hold that same feeling.
Drupal as opposed to the other CMS products is also recommended to clients that are more technically savvy and understanding of this type of development environment. For example the Drupal documentation tends to have gaps and missing information however this is easily made up via 3rd party authors who have written. Drupal may be the right selection for you if your are building a web product or service from the ground up or looking to integrate with another product that is based on Drupal such as xTuple.