My favourite language for the past several years has been Python due to its depth and its breadth. Lately I have recently worked with node.js and I am very impressed with its capabilities and flexibility. I use my own build of Python that is portable (to Linux OSes) and entirely self-contained.
Lately I've been coding an application and some tools that make use of AMQP messaging, in particular using RabbitMQ as the MQ Broker. I am very impressed at how AMQP, combined with network db APIs, lets you build an application in many small decoupled components that can easily be scaled on one or many servers.
Since RabbitMQ is implemented in Erlang, I learned enough Erlang to make a simple management app that runs on an independent Erlang node, i.e. not a RabbitMQ plugin. And since my company's main language is Java, I am now learning Scala (with Akka actors) since this JVM language has part of its heritage in Erlang.