Ruby on Rails was the first major web framework to include automated tests as part of every new project. The effects have been enormously positive. Many developers have learned to write tests well, which in turn helped them get better at design. Experienced developers were drawn by Rails' focus on productivity and beautiful code. Their community activities have helped push the craft of web development forward. Over the years positive effects have rippled to other communities too.
In practice though, how much do Rails developers really write tests?