You may have heard of DXA, SDL's Digital Experience Accelerator. How does that relate to DD4T? Read all about it here.
DD4T (Dynamic Delivery for Tridion) enables the use of MVC frameworks in Java and .NET with Tridion. At its core, DD4T publishes Pages and Components as data into your Content Data Store rather than as code to your filesystem. You can then leverage tools which you know; such as ASP.NET MVC, Spring - or anything that can use REST to deliver your content in any view you like.
DD4T is built by the community. So, we depend on people's own time to help develop new features and fix bugs. You can help too!