The first real world java application I made the s390 cacao port run was my favorite one, jython:

$ ~/cacao-dev/inst-s390/bin/cacao -jar jython.jar
Jython 2.2a1 on java1.5.0 (JIT: null)
Type "copyright", "credits" or "license" for more information.
>>> map (lambda p: java.lang.System.getProperty(p),
... ["os.arch", "os.name", "os.version"])
['s390', 'Linux', '2.6.17-2-s390']