If you have a legacy Java project that is a Big Ball of Mud with no unit tests, you know that the first step in refactoring it is to get some unit tests in place. But instead of writing those tests in Java, take advantage of the fact that Java and Groovy both ultimately compile to the same byte code .class files. By writing your tests in Groovy, you write less code, and the tests are clearer to read during code review sessions.
The presentation is here https://www.assembla.com/code/SampleCode/subversion/nodes/presentations/groovylegacycodeandyou?rev=223 in your choice of ODP or PDF format.
Scala and Clojure get a lot of press, and they should because they are great languages, but we need to recognize Groovy for being a real workhorse that can help wean us off of pure Java code development, and get our projects under control applying SOLID principles and TDD.