To Java Programmers
If you program for the JVM, you owe it to yourself to check out Scala.
Scala is a BSD-licensed language that combines imperative, functional, and object-oriented programming. It is a compiled language that targets the JVM (a .NET version is in-progress). The syntax deliberately mirrors Java, in order to make it easy for Java programmers to learn.
One of the main attractions is that problems can be solved with shorter, more maintainable code sets. Functional programming also shows great promise for parallel and distributed programming.
Several major companies now have large-scale Scala projects, including Twitter.
(In many ways, Scala is a JVM equivalent to what Microsoft is doing with F# for .NET -- also an excellent language, and now open source as well).
In any case, the homepage for the language is:
You can build a prototype by the book, but a legend you build by the seat of your pants.