There's a list of over 80 languages for the JVM at (
http://en.wikipedia.org/wiki/List_of_JVM_languages). I don't think you can really recommend any. The article says:
Apart from the Java language itself, the most common or well-known JVM languages are:
Clojure, a functional Lisp dialect
Groovy, a scripting language
Scala, an object-oriented and functional programming language[1]
Kotlin, statically typed programming language compiled to JVM byte code and JavaScript by Jetbrains
Ceylon, an upcoming programming language and SDK, created by Red Hat
JavaFX Script, a scripting language targeting the Rich Internet Application domain (discontinued 2010)
JRuby, an implementation of Ruby
Jython, an implementation of Python
Rhino, an implementation of JavaScript
AspectJ, an aspect-oriented extension of Java
I would add Gosu, Fortress/JVM, and Beanshell as important. Others were talked about at the recent JVM Language Summit (http://openjdk.java.net/projects/mlvm/jvmlangsummit/agenda.h...).