On the client-side Flowdock is written in JavaScript (programming language). It uses jQuery, jQuery UI and some other libraries, but for example the Comet (programming) implementation is our own.

We have a messaging backend written in Scala (programming language), running a Jetty (web server) server. The actor model suits our use-case pretty well, and Akka (http://www.akka.io/) implements them pretty well. The messaging backend also acts as a SMTP server accepting incoming emails in realtime, connects to the Twitter (product) Streaming API etc. Messages are stored in MongoDB.

All the easy stuff (the website, user management, payments, admin UIs etc.) is powered by Ruby on Rails (web framework).

Our Linux servers are managed using Chef (http://wiki.opscode.com/display/chef/Home).