Some people ask if Test Driven Development (TDD) is what caused Ruby to become popular. Other people smarter than me claim that TDD is in fact the reason that dynamic languages are now viable options.
I'm sorry, but I disagree.
Can you write a non-trivial application in Ruby without tests and have confidence in it? No. Can you write a non-trivial application in Java without tests and have confidence in it? Again, no.
A compiler lets you know that some things are correct, but should not give you confidence that your application behaves as expected. A trivial example is, I need to know that the calculate_tax (that's calculateTax for you Java fans) method returns 56, not that the result is a Fixnum (or Integer in Java).
As much as it should be, TDD is not mainstream. There's no TDD Conference, but RailsConf sells out every year.
I would actually credit David Heinemeier Hansson with making Ruby viable by creating a framework that drove mass adoption. But, I bet of you looked at the majority of Rails applications you would find empty test folders (or only the generated tests, which are never run). I'm quite sure that's true because I expect the conferences to attract the best of the Ruby developers, and several of the people I talk to at those conferences "simply don't have time" to write tests.