WebSync is a full-blown comet server built for Microsoft stack (IIS/MSSQL/.NET). It’s written in 100% managed C#, and uses IIS to reach incredibly high user concurrency. Despite being written in C#, we’ve made it easy to integrate other languages as well by pre-building client libraries and allowing generic HTTP requests to send messages. It’s a pretty neat product, easy to get started with, so check it out including demos or tutorials.
More info: WebSync