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
- Theme Test Drive allows you to test out new themes without activating it and showing it to all of your blog visitors