A comparison of the capabilities and features of the latest releases (as of September 2008) of Joomla 1.5, Drupal 6, and Wordpress 2 with respect to 1) functionality -- multi-user publishing, layout and design, search engine optimization (SEO), mobile device support, and internationalization/localization; 2) extensibility -- the general climate and quality of third-party extension development for each platform; 3) support; 4) specific kinds of websites -- media/publishing sites, community/social sites, eCommerce sites. This is a helpful breakdown when it comes to deciding which platform is the best fit for a specific purpose.

Addendum: Please take note of the date of this document, which may or may not be updated. Open Source software changes--overwhelmingly for the better--every second. Some caveats, corrections, and deeper discussion of the "big three" CMS comparison topic can be found at the Joomla forum, where this thread motivated me to produce this document and then popularized it. Discussion has continued at the Joomla forum since then, as well as my blog in posts like this.