For anyone that's interested in alternative programming languages, I've written a lengthy introduction to Clojure's agents.
For those unfamiliar with Clojure, it's a modern Lisp running on the JVM with excellent support for concurrency. It allows excellent interoperability with Java, so essentially you get a dynamic and clean programming language with a huge standard library. On the performance front, it's often on par with Java. I've spent a good deal of time learning the language lately, and I have to say that it's very enjoyable to work with.
One huge bonus of working with Clojure inside Vim is the VimClojure project. It allows interactive programming from inside of Vim and proxies code between a stateful NailGun server and the editor. To be honest, I wasn't aware that an interactive REPL was possible inside of Vim until I installed this, and I'm wondering if there are any other projects out there using similar techniques with different languages.