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