One cool feature about Ruby on Rails is that clean URLs is something that comes natural. Rails uses clean URLs by default. I love this. This is because I have suffered a lot with Drupal trying to make it use clean URLs. I also found it needed some work with PHP trying to make it use clean URLs too.
The W3C web site uses clean URLs in all its web pages. One interesting article in the W3C about clean URLs talks about this. The article though comes under the name Cool URIs.