JMP library allows explicit multiprocessing in Java with API that resembles Java Multithreading which is an essential built-in Java feature. JMP allows easily running a piece of code on a separate process.
JMP also contains a simple cluster management tool.
Two of the most obvious JMP use cases are:
a. All kinds of data parallelism (exhaustive search algorithms, optimization problems).
b. Server-side load balancing between available CPUs.
See project Wiki for more.