Alternative JVM languages
I am enjoying Martin Odersky's course "Functional Programming Principles in Scala." I use JRuby and Clojure a lot in both consulting projects and my own projects but my main use of Scala in the past was writing some programming examples for my book "Practical Semantic Web and Linked Data Applications, Java, Scala, Clojure, and JRuby Edition" that is available here for free. I had purchased three Scala books and occasionally played with Scala but it never really "clicked" with me (same situatuon with Haskell: bought three books on Haskell, lots of experiments, never had the "clicked" experience).
Taking Martin's class is definitely helping me become more comfortable with Scala.
Having an interactive repl for Clojure and JRuby has always been a big win for me over Java development, even with great Java tooling (e.g., when I have to do GWT or SmartGWT development, having both the server and client side code in Java, all debuggable in IntelliJ is nice, but still a very heavy weight development environment!)