Last year I interviewed at two YC companies, AirBnB and Heroku. At AirBnB the interview went great, but then I was given a 3 hour programming challenge that including a written analysis of my code. The challenge was not at all related at all to building web apps, and even the use of a database was not allowed to be used to solve the problem.
At Heroku, we discussed a real issue they were having and halfway through the interview we were both up at the whiteboard bouncing things back and forth, drawing things up. At the end of the Heroku interview we both knew how the other worked and how we would work together.