Glimpse is a Firebug like debugger used to debug ASP.NET. Among others it can help you debug routes and enable tracing in ASP.NET MVC, it also has a plugin to help you debug Ajax. Glimpse also allows you to take a look at the execution stack, including all action filters and etc. called to display the view or page.
At the moment Glimpse supports ASP.NET Web Forms and ASP.NET MVC, but more technologies are going to be supported, including PHP, Ruby on Rails, and Node.js.
It is a very powerful tool and enables you to debug stuff, which can be difficult to debug using Visual Studio.