I've been referring to Clojure as one of the best Java libraries bc it really requires you to write Java in pig Latin a lot of the time.

My favorite Java library: Clojure.

My favorite C# library: ClojureCLR.

My favorite JavaScript library: ClojureScript.

My favorite Clojure library: CommonLISP.

Edit: As an aside, just goes to show how powerful of a beast PCJ really is - very underappreciated.