Look at the key differentiator of each language when it was first created. So,
If you really need the JVM...
* Groovy for quick-to-write dynamically-typed scripts for testing Java classes
* JRuby if you need Java 7 and invoke-dynamic
* Scala if you need static compilation
* Clojure if concurrency is important
Of course, if you need specific software, choose Groovy for Grails, Scala for Lift, Clojure for Leinengen, etc.
If you don't need the JVM, consider Ruby, Python, Javascript, etc.
Look at the key differentiator of each language when it was first created. So,
If you really need the JVM...
* Groovy for quick-to-write dynamically-typed scripts for testing Java classes
* JRuby if you need Java 7 and invoke-dynamic
* Scala if you need static compilation
* Clojure if concurrency is important
Of course, if you need specific software, choose Groovy for Grails, Scala for Lift, Clojure for Leinengen, etc.
If you don't need the JVM, consider Ruby, Python, Javascript, etc.