DD4T (short for Dynamic Delivery 4 Tridion) is a light-weight framework which links Tridion to your MVC web application architecture. It is available both for Java (Spring MVC) and .NET (ASP.Net MVC 3).
DD4T exposes the pages and components that you publish from Tridion in a simple object model, allowing you to do 'templating' in MVC views. Instead of writing Tridion templates that generate Java/JSP or .NET code, you can create an MVC web application in your favorite IDE, using Tridion components and pages as your model.