title: Open source projects that demonstrate TDD and SOLID priciples
body: <p>I asked a similar question before, and got some good answers, but I think it was too general.</p>
  
  <p><a href="https://stackoverflow.com/questions/559484/examples-of-great-software-design-and-implementation">Examples of great software design and implementation</a></p>
  
  <p>Does anyone know of any open-source projects that demonstrate really good TDD practices, and SOLID principles?  TDD and SOLID are widely publicized, but I've never seen it really done myself, I'd just like to get an idea of what it might look like in a project (large or small)?</p>