I was trading e-mail yesterday with the legendary Ted Nelson. Xanadu is a concept more ambitious than the World Wide Web, and has been struggling toward realization for decades. Along the way, it has spawned a number of interesting technologies, among them enfilades, the Ent, and, quite recently, a product called ZigZag.
Enfilades are a class of data structures and their corresponding algorithms.
Beyond that, I am at a loss to say what they are, except that they are an important part of Xanadu. The Ent is a data structure plus algorithms to manage complex versioning of arbitrary objects with low overhead and great parsimony. Complex versioning is a feature of Xanadu. ZigZag is a shareware program that runs on Linux or any platform that supports Perl 5. It is a hyperstructure kit, but you can download it and figure it out for yourself.