ASP.NET 5 is a landmark release in many ways. It’s full open source from the start, it’s cross-platform and supported on Windows, Mac, and Linux. It includes contributions from the community and is being developed in the open on GitHub.
ASP.NET 5 includes lots of great updates as well like TagHelpers, the unification of MVC and Web API, the componentization of the .NET Framework, dependency injection built in, an all-new project system, and deep client-side tooling for things like Bower, Gulp, and Grunt. This is an exciting time for the Microsoft web platform.