JS front-end frameworks and libs
Ext Js: powerful MVC and MVVM framework for desktop like UI on the web using mostly declarative programming.
jQuery: no need to say more
Underscore.js: library with utility functionalities, binding, applying, map, reduce, all, each, etc.
Meld: Aspect oriented programming for JS.
Backend frameworks
ASP.NET Web forms, MVC: MVC is the future.
ASP.NET Web API
Zend framework
Other libraries and technologies
NHibernate, LINQ, Entity Framework: ORMs
Dependency injection: Spring.NET, Spring for Java, Unity.