Groovy is by far my favorite scripting language, since it leverages and integrates with all the Java stuff I use (applications, tools, server software, etc). It is just as expressive and powerful as Ruby, Python, etc., but feels a little simpler or cleaner, depending on the comparison.

Here are two fantastic resources in the ‘Cookbook’ fashion, showing how to do many useful things with Groovy:

The Groovy website Cookbook
PLEAC’s Groovy Cookbook – a Cookbook similar to Perl’s original Cookbook.