datapile uses Scala
Scala is the God of languages. A legend. The Mount Rushmore of hybrid OO/functional languages is Scala's face four times over.
Ok, honestly, we love Scala. We love(d) Java (and it's parents C and C++), and we love(d) all the languages that borrowed cough stole cough from Java over the years such as Groovy, Clojure, and C#.
It may not be perfect (it totally is, but since programming languages don't have egos of their own, we don't want to paint it too bright), but it is awesome. It runs on the JVM, you can utilize Spring, it works great for data processing (which is sorta kinda the thing we do here, folks), and it just makes sense at all levels.
If you don't like Scala, we feel sorry for the projects that are suffering due to your choices, meanwhile we are using Scala to write everything from JavaScript, CSS, SQL, and JSON directly within itself (go figure), so in the end no one will know the beauty of this powerhouse language (except for our engineers, of course).
Foursquare uses Scala
Nearly our entire server codebase is written in Scala (if you haven't heard of it, it's a programming language that is basically what you would get if Java + ML had a baby). This has worked out super well. It enables us to write concise easy to deal with code that is typechecked at compile time. It's also been a big help with recruiting.
papaver uses Scala
worked with scala for around 2 years. really enjoyed the language and getting back into the world of functional. unfortunately the community is heavily fragmented and the language itself broken and inconsistent. that with the various factions involved made it a put of for long term investment.
elo80ka uses Scala
Scala, Akka and Spray (which became Akka-Http) provided the building blocks for the menu service.
Akka's actors and finite-state machine were a natural way to model a USSD menu (a series of stateful interactions between a subscriber and the USSD gateway).
giovannicandido uses Scala
Replaces entirely the Java Language to build a much more expressive and powerful code on the backend, while leveraging at the same time the Java Platform Tools and Frameworks, is a mixture of old and mature with new and sexy.
Oomba uses CoffeeScript
All of our Javascript code is first written in CoffeeScript for ease of reading / writing. It is compiled to Javascript before being minified and served to the client.
opening.io uses CoffeeScript
All front-end / back-end is driven by Coffeescript. For the main ReactJS functionality JSX is embedded with coffee in .cjsx files / handled by Browserify.
jflynn33 uses CoffeeScript
We like CoffeeScript because it's more readable, we use it because we have a lot of libraries and functions already (plays nicely with Rails, too)
Tinkhaven uses Haskell
Backend logic (REST, Auth0, Redis, Postgres, functional graph library (fgl), ...)