There are 3 very popular CMS options available to you and all have large developer communities and a wide array of free plugins, modules, and themes.

The 3 in order of size used on the web are WordPress, Joomla and Drupal. Ease of use follows the same order, as well as power and strength.

WordPress
WordPress started as a blog CMS with roots dating back to 2001. It offers plenty of plugin's and themes to extend your website. The beauty of WordPress is the ease of use for the end user. It is the most popular of the open source solutions on the market has a huge developer base and lots of forums and help information about the CMS.

WordPress Pros
€Great for small business
€Ease of use
€Lowest cost of ownership of the big 3
€WordPress has decent SEO (Search Engine Optimization), it gets you 80-90% of the way there out of the box. With some additional work and plugins, it will help your content's visibility on search engines.

WordPress Cons
€Security Flaws & Attacks. Being the largest open source CMS, it's the favorite of hackers, much like Microsoft. Topple the big boy on the block! The good news is WordPress is patched quickly when a compromise is found.
€The architecture of WordPress is as a Blog platform. You cannot natively define multiple content areas within the template.
€Not as developer friendly as Joomla or Drupal

Joomla
Joomla is another popular option first released in 2005 and was a spinoff of the Mambo project. Joomla has gained a lot of interest and with the release of the 3.0 platform, has become more viable than ever before.
Over half a million members worldwide, puts Joomla as a leader in the world of content management solutions. With over 9000 Plugins and Modules it very easy to extend your site to include forums, ecommerce and almost anything else you dream up.

Joomla Pros
€Used in a lot of high profile sites
€Friendly for all types of users, including designers, developers and administrators
€Very scalable
€Large developer community with tons for support
€It is very easy to create and operate a multilingual website with Joomla.
€Version 3.0+ has ACL and Security system allows for a better access control mechanism over previous versions

Joomla Cons
€Takes Time to Learn
€Certain changes may require the assistance of professional developers

Drupal
The big boy on the block has a large following and is a very powerful, if not the most powerful solution in the open source world. Drupal has over 14,000 modules and nearly 1,500 themes.

Pros
€Very flexible for adding custom features
€Highest level of security of the big 3

Cons
€Drupal is getting better, but it's much harder to install, configure and customize
€Administration and Content Management is not easy and not very user friendly
€Getting a Drupal website published can be more time consuming, which translates to increased cost over WordPress or Joomla

As you can see there is no perfect open source content management system. The best approach is to define what your site is going to do and then look at the options available to you. After you decide what you want, talk to a professional developer.