Groovy is a dynamic object oriented language for the JVM.
Ranked in these QuestionsQuestion Ranking
Pro Integrates well with Java
This link adds some interesting points explaining how Groovy integrates better with Java than e.g. Scala. If you want to write classes that can be consumed by Java classes, or easily consume existing Java code, Groovy is probably a pretty wise choice. It can be statically compiled and supports optional typing, which makes it much more maintainable for larger projects than e.g. JRuby.