Most of our code is in Scala and most engineers use the IntelliJ IDE. We use a handful of akka modules including HTTP, Streams, and Aplakka.
We run our production environment on AWS, with a bit of redislabs, CloudAMQP, and Heroku as well. Most of our stack is deployed straight to EC2, but we're increasingly moving to Docker and kubernetes. We have a few hundred terabytes of Elasticsearch running in self-managed clusters on EC2.
We move a lot of data around in messaging systems. Today that is largely RabbitMQ and Kafka, but we are experimenting with Apache Pulsar which we hope will replace both of these systems. Pulsar promises better performance and uptime than RabbitMQ, while allowing many more independent data streams than Kafka.
On the Frontend, our engineers work primarily in Javascript(ES6+) with ReactJS and redux-saga. We transpile our javascript using babel, and we use webpack to bundle our assets. For styling, we use postcss to preprocess our styles.