WordPress is extremely extensible due to the wide variety of plugins available

A few useful plugins:

• Feed WordPress allows you to incorporate other articles into your own blog using RSS
• WPTouch transforms your existing WordPress theme into a web-application experience when viewed from an iPhone, iPod touch or Android touch mobile device.
• Daggone Sitemap Generator
• Simple Tags
• FAQ-Tastic
• Theme Test Drive allows you to test out new themes without activating it and showing it to all of your blog visitors