It's not the nicest code you'll ever see, but it's a great demonstration of the power inherent in Ruby on Rails. Miklos Hollender demonstrates how he created a version of Web 2.0 app, Reddit, in under 20 minutes using Rails, scaffolding, and a little elbow grease. Source is included.

An interesting aside based off of this is an interesting video demonstrating how to recreate Reddit in Lisp in 20 minutes, as Lisp is somewhat a cultural uncle to Ruby, it's worth watching.