Microsoft announced today that they're preserving a bit of history here - with Zork I, Zork II, and Zork III now officially and clearly open source. The source has been around for a while but now it's all proper.

From the announcement they said:

Today, we’re preserving a cornerstone of gaming history that is near and dear to our hearts. Together, Microsoft’s Open Source Programs Office (OSPO), Team Xbox, and Activision are making Zork I, Zork II, and Zork III available under the MIT License. Our goal is simple: to place historically important code in the hands of students, teachers, and developers so they can study it, learn from it, and, perhaps most importantly, play it.