Q:

Java Developer looking for a 2nd'ary language to play with

What is best language to learn next to Java? 
Criteria for this secondary language are:

High potential of being the "next big thing".  e.g. If the market for Java open positions hypothetically dies/dwindles, what is the next programming language that will have a bigger market for open positions?  Another way to frame this question is: If I own a small company that implements solutions in Java, what is the other language that I should use? 
Can produce web applications. 
Can produce desktop applications. 
Easy and fun to learn. 
Wide range of available libraries and frameworks (free or open source) that enhance and speed up your solutions. 

A:

Python almost meets all of them, but I don't know about being "the next big thing", but hey, Google uses it, and I think its popularity is raising. 
It's a scripting language, btw. 
I use it for web applications (using django), and you can definitely create desktop applications with it (although I haven't done that myself).  
It is easy and fun! ( although this is quite subjective, but it's tons easier and "funner" than Java)

A:

For employability: Any of the .Net languages, probably C#.  Then you're well set for most potential customers. 
For stretching yourself: something functional (F# to cover .Net too?),  or something Lisp, or Smalltalk - was once the next big thing but it probably never will be again, but still a language that changed signficantly my approach to programming in other languages. 

A:

If you are expert in java, you will probably really enjoy groovy.  
It is backwards compatible with java and has a lot of the nice features of ruby and similar dynamic languages.  It also has what is shaping up to be a nice rails-a-like in grails, and a good object-relational mapping framework in GORM. 
What is really nice about it is you can mix and match groovy classes and java classes.  So if you find yourself forgetting the groovy syntax, you can just drop into Java.  And of course you can get at any Java library from it.  At the same time you can benefit from groovy's really concise idioms for things like Beans properties. 
(By the way it meets all your criteria listed above, too)

