Softminer.net

Here is a good example of publish subscribe design pattern in c#
http://www.codeproject.com/Articles/34316/Topic-based-Publish-Subscribe-design-pattern-imple

Here you can read about Publish Subscribe design pattern
http://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern

WCF has already some method to implement it
http://www.codeproject.com/Articles/34333/Topic-based-publish-subscribe-design-pattern-imp