Microsoft ASP.NET MVC
There are a number of reasons to be excited about this framework. Some of the biggest are separation of concerns, testability, and performance.
Separation of concerns allows you, the developer, to create self aware components that don't heavily depend on another un-related component to function. Think about removing the logic of your postback event in ASP.NET 2.0. Take that code and move it somewhere more centralized that will be used by similar web views. That concept is the relationship between the Controller and the View. The View should simply display something while the Controller should tell it what to display. The Model describes the data and is used by your Controllers and Views as something I like to think of as a transitional object.
Testability is a major concern with most developers these days. Currently automated unit testing is nearly impossible with the current Web Forms architecture present in ASP.NET. Sure, you can unit test your data and business layers. However, once these layers reach the presentation layer, testing expected behavior just isn't possible in an automated fashion. The ASP.NET MVC framework allows testability through the separation of concerns along with the way a Controller will dictate which View is rendered. That functionality allows for easy testability.
The final reason I'll discuss here is performance. ViewState is a piece of magic all too often by the Web Forms architecture and simply is a bloated method for persisting data across HTTP requests. The ASP.NET MVC framework will remove this dependency and will provide better performing environment for our web applications.
I'm in the planning stages of a new project and really investigated whether I should begin ASP.NET MVC development. Since it's so new I'm not going to risk it but I encourage everyone to keep an eye on this project. I'll definitely be doing that and will be all over a release closer to RTM.
Resources:
ScottGu - MVC Tag
Phil Haack - ASP.NET MVC PM