Dropbox is a file hosting service that offers simple cloud storage and file synchronisation for average users as well as big companies. From a humble start in 2008 it grew faster then anyone could have predicted and hit 200M users on Nov 13, 2013. Even though it has many competitors like Google Drive, Microsoft SkyDrive and Box.net (and many, many others), Dropbox continues to stand at the top of the game.
Where is Python used?
It would have been virtually impossible to do Dropbox without Python
-- Rian Hunter, Dropbox Engineer
The better question would be - where isn't it used? More then 99% of both the server and client code is written in Python. Using Python allowed the engineers to be able to quickly test their ideas, fix any problems and bring frequent updates. They were lucky, as many of their Python solutions scaled surprisingly well, which meant that "(they) could get to 40M users without having to write thousands of lines of C code", as the whole platform was still running only a few hundred lines of Python code even when the number of users started reaching tens of millions.
Rian also said that every time they tried to anticipate things or "be clever in advance", they failed - it was much easier to just stay on top of the architecture as it grew and keep tabs on it, which probably wouldn't be possible if they weren't using Python.