Drupal is a solide Web content management system that allows you to develop complex websites with literally thousands of community-contributed modules. With well conceived Web content management systems, Drupal separates content from presentation through the templates (PHP, xHTML and CSS).
Drupal: Powerful Engine for building up your favorable Web Applications
 
Key Advantages

• Content authoring with flexible access controls
• Macintosh and Windows support for content editing
• Forums, blogs, newsletters, wikis, quizzes, polls, sweepstakes, and other social networking modules
• Gallery/slideshow module
• Calendar/event module
• Built-in caching
• User registration and profiles
• Out-of-box RSS feeds
• Role-based permission system
• Browser-based administration
• and well over 1,500 more modules!
• Decentralised Editing on any browserh4>
• Friendly URLs
• Permalinks
• Taxonomy and Tagging
• Google Ajax Search Module
• Platform Flexibility