Back End Developer
ASP.NET MVC
Website: http://www.asp.net/mvc
Source code: http://aspnetwebstack.codeplex.com/
Most of our application runs on top of ASP.NET MVC, so for me it's really important to know it. I think it's a good framework, with a lot of extensibility points, good implementation of the MVC pattern, and we use it in heavy traffic projects without particular problems. Of course there are few things I don't like of it...