My solution to the Bank kata on Codurance's Katalyst, an outside-in TDD challenge.
After about an hour and a half, I could achieve a very simple, yet satisfactory solution. The three classes are short and almost 100% covered by unit tests.
This kata is very good at showing you how TDD is not a testing practice. Rather it's a design discipline that helps you achieve a simple design by pushing YAGNI and a very short feedback loop.