Joomla, Drupal, Wordpress, or ExpressionEngine?
officemedium: Drupal, especially if you have any development experiences or real customization needs. It can be modeled to do anything - yes, I mean anything at all. I've developed some amazing applications using Drupal - not just the typical 'blog' like site that the others generally create. So it depends on your needs. Drupal also has, what I think, is the best community out there - tons and tons of modules, themes, support, customizations, tutorials, etc.