ZeroMQ at Spotify
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This article from PyCon was titled "How we use Python at Spotify" but I wanted to focus on the fact that they also use ZeroMQ. It's a little difficult to find a list of the big-name companies that use ZeroMQ, but now you can rest assured that one of the biggest fish in the IT pond use it.
The other two major use cases of ZeroMQ that I know about are CERN and Loggly. I'm sure there are more though.