This is a huge open-source contribution from Microsoft: http://www.codeplex.com/pubsub.
The Distributed Pub/Sub Event System is a solution for cross-process and/or cross-server scenarios where a large amount of event handshaking is required and low latency is important. Basically, this architecture was designed for speed.
Even if you can't think of a usage scenario for this code-base in your stack right now, I strongly encourage you to just crack open the source and take a look. I personally think the design is a prime example of true software engineering craftmanship, which is simply a thing of beauty.
By the way, I was the one who tested this bad boy before it was released. Therefore, I can't claim rights to the elegant design, but I can claim rights to some of the quality--hopefully not the lack thereof!