I needed some sample code for an interview, so I built a website called Blooger. It's a site (like Blogger) where people can go create a blog to blog about their booger stories. It even has functioning Atom feeds :)
Here's the source code. I used Cucumber, RSpec, Webrat, and factory_girl to do behavior driven development. I used Authlogic for authentication. I used BlueCloth for Markdown support in order to format the blog posts. The README in the source has more details.