Decoupling your architecture

Messaging Techniques

Spring Remoting
Remote Method Invocation (RMI)
Spring's HTTP invoker
Hessian
Burlap
JAX RPC
JMS

RabbitMQ
ZeroMQ

REST

Naming Servers
doozer

Abstraction on top of messaging
Camel abstracts away your messaging layer so you can just call methods on services and if you route everything properly.
Spring Integration also does this but we had issues with the way it considers a service only a method while we wanted an
entire class to represent a service with multiple methods

Serialization Protocol
Java Serialization only works in java land and not in a polyglot system.
Protocol Buffers seems to fit nicely here