Building a Ruby Gem (From Scratch) Using Behavior Driven Development
If you read a lot of Ruby blogs, you might see people talking about testing (or its behavior driven equivalent) as if it's the holy grail, yet most Ruby books and online tutorials fail to cover it in much detail at all. Last year, Jamie Van Dyke wrote an article for The Rubyist called Building A Gem Using BDD to put things right (the article was licensed exclusively to the magazine until recently).
In the article, Jamie walks through creating a Ruby library (to ultimately be distributed in gem form) from scratch using behavior-driven development (BDD) techniques (specifically, with RSpec). If you're not a regular tester and RSpec just seems like a buzzword to you, Jamie's tutorial is a great way to get your feet wet with the concepts because it's code-driven and you get something that works pretty quickly.
As an aside, Jamie was interviewed by FiveRuns for the TakeFive series back in September 2008.