Ask HN: What is the most cost-efficient COMPLEX web app you have seen?

I am looking for stories basically. So please don't start a philosophical fight over the definition of "complex" for example.

And it doesn't matter if you created the architecture of that web app or someone else, please share it.

I guess most people would be interseted in architectures that use cloud services, because not everyone can run their own servers or manage everything on their own.

Thanks