List of SEO Tools - Content Management Systems

Joomla - This is a free, open source content management system. It reaches the level of an enterprise content management system in terms of features. As you'd expect from an actively used and developed open source piece of software, it has thousands of free extensions and add-ons.
Its down side is also as you would expect for open source software. If it gets buggy, there's no customer service people for you to take your complaints to and ask for help.

WordPress - Simply put, this is the best known blog platform on the Internet. WordPress is capable of producing search engine friendly pages and has tons of plug-ins.

This blogging platform has been around for quite a while -- more than six years, in fact -- so it's easy to find articles that explain how to set it up for your particular purposes. Also, since it is so popular, many major web hosts support its use. The best part is that it's free.
Open Source CMS (http://www.opensourcecms.com/) - Here you can find tons of free content management systems of all kinds. You're quite likely to find one that will suit your needs and the needs of your web site perfectly, or close to perfectly.

Keep in mind, however, that choosing a content management system is an important decision; it will affect everything you do (and CAN do) with your web site. So get some feedback on forums before getting one, because once you have content within a CMS, it's hard to change anything.