Zork I, II and III have been released under the MIT license.
You can find it on github.
I loved these adventure games so much. It's why I was excited to work on Warcraft Adventures...until I saw the code.
@JeremiahDJohns Forward to the 2nd ed:
"But I cordially dislike allegory in all its manifestations, and always have done so since I grew old and wary enough to detect its presence. I much prefer history, true or feigned, with its varied applicability to the thought and experience of readers."