Have you used Groovy outside of the Grails framework? If Yes, in what circumstance was Groovy useful?

Groovy its an easy to learn and use scripting language for Java Virtual Machine (JVM) environment. Groovy is aimed at increasing the developer productivity by writing less code don't repeat yourself (DRY). You can use Groovy to develop Desktop, Web and Mobile applications.

For desktop application development I recommend checking the Griffon framework. It's inspired by the Grails framework.