Trend Micro System has released its Trend Message Exchange(TME) source code with "Apache License". If you are interested in this system, you can see the slides for introduction or go to the official web site.
As you see in the architecture, TME provides the features of horizontal scalability and stability. In the past years, if we want to develop this kind of system, we should consider the distributed system design, having to write a lot of codes and do much more testing for the stability reason. But now, we can take an easy way to establish the same kind of system without charge.
Are you anxious for constructing a distributed and scalable message system? Let's enjoy this new technology!