JetBrains MPS is a language development environment.
You can download the last stable version from here
What is Early Access Program (EAP)?
We at JetBrains believe that making good tools for developers should greatly rely on developers' opinions. Our Early Access Program lets the development community closely participate in discussions devoted to JetBrains MPS and influence the planning and development from the early stages onwards. We hope this EAP will be both valuable and exciting, just as EAP program for IntelliJ IDEA is. We highly appreciate your feedback and ideas!