Context Sensitive ProgressBar using Long Polling / SignalR /

Asynchronous scalable web applications with real-time persistent long-running connections with SignalR

http://signalr.net/

There are couple of nice articles published by "Dino Esposito" at MSDN Magazine. Here are the references to them

A Context-Sensitive Progress Bar for ASP.NET MVC

Enhancing the Context-Sensitive ASP.NET MVC Progress Bar

Long Polling and SignalR

Also following are few more references

Asynchronous scalable web applications with real-time persistent long-running connections with SignalR

ASP.NET MVC 3 Real Time Collaborative Apps with SignalR

Fun with @SignalR