DD4T is a light-weight framework which links the SDL Tridion CMS to your MVC web application architecture.

It is available both for Java (Spring MVC) and .NET (ASP.NET MVC 3, 4 and 5), and it supports Tridion 2009, 2011 and 2013.

Downloads
Template Installer (1.31)
https://code.google.com/p/dynamic-delivery-4-tridion/downloads/detail?name=DD4T-template-installer-1.31.zip&can=2&q=

Visual Studio template (VS2012/2013, Tridion 2013 + MVC4) https://code.google.com/p/dynamic-delivery-4-tridion/downloads/detail?name=DD4T.1.31-MVC4-Tridion2013-VS2012-VS2013.vsix

Official NuGet packages
DD4T-Framework https://www.nuget.org/packages/DD4TFramework/
DD4T-MVC3 https://www.nuget.org/packages/DD4T-MVC3/
DD4T-MVC4 https://www.nuget.org/packages/DD4T-MVC4/
DD4T-Tridion-2011sp1 https://www.nuget.org/packages/DD4T-Tridion-2011sp1/
DD4T-Tridion-2013 https://www.nuget.org/packages/DD4T-Tridion-2013/
DD4T-Tridion-2013sp1 https://www.nuget.org/packages/DD4T-Tridion-2013sp1/

If you plan to create a .NET web site with DD4T, start with the .NET Quick Start https://code.google.com/p/dynamic-delivery-4-tridion/wiki/41_QuickstartDotNet or this blog post: http://blog.building-blocks.com/technical-tips/getting-started-with-dd4t-1-31-asp-net-mvc4-and-sdl-tridion-2013

There is also a Java Quick Start available. https://code.google.com/p/dynamic-delivery-4-tridion/wiki/31_Quickstart

If you want to get involved by collaborating. We follow the forking workflow. Fork the repository and submit a pull request for it to be merged in.

For more information on the Forking Workflow, see Atlassian's excellent article.