Here is what I find useful in commons-lang:
What a shame that we cannot add methods on existing classes in Java. Even though StringUtils has lots of methods, it seems to always lack some. In Ruby, I could just open the class definition and add new methods. I am not sure I would like that in Java (and it will never be able to, I know).
But C# in .Net 3.0 has something more controlled and type-safe. They have extension methods.