Xanadu is a project by Ted Nelson.
Ted Nelson is a fundamental figure of early computing (despite not being a computer scientist himself). His work (recommended: Dream Machines/Computer Lib, 1974; Literary Machines, 1981; Geeks Bearing Gifts, 2008) is about how computers touch us as humans, and how they affect our future, especially when it comes to communicating and using them to spread+build knowledge and ideas. When Nelson wrote his first essays, very few people had the insight or knowledge to really understand what he was going on about. Nelson came from a movie acting/storywriting/literary background, and this allowed him to establish parallels and formulate ideas that no one at the time could have done. I would argue that even today, some of his finer points still hold deep relevance and are yet unknown from the general computing crowd.
Ted Nelson remains a fairly unknown figure in computing, partly because he never wrote much code/developed products/worked in traditional research groups himself (a major misstep in a field where you tend to be mostly recognized for the projects you ship or papers you publish), and because a lot of his ideas were replaced by ideas that are much more simple to implement and make viable. A lot of hackers do like to ridicule him for those reasons, which I think is a mistake - despite those shortcomings, his body of work is still full of ideas and theories that you won't find anywhere else. You can almost see a second, parallel path that computing could have taken in the 70s. (Nelson was respected by many more famous computer scientists, including for example Alan Kay)
That's where Xanadu comes in: Xanadu is an implementation for Nelson's vision for hypertext (which predates the hypertext we know by more than a decade or so). His hypertext is much deeper than Berners-Lee's: notably, it is based on 2-way links (so not only can a page link to other pages, but you can also see which pages link to that page), a deep version system (so that you can access past versions of webpages), stronger "linking" (no such thing as "broken links" in Xanadu), etc. Of course one can see why this is much harder to implement at scale than the web we have now. Nonetheless, Nelson has been working on it through the years, as evidenced by this release.
PS: if you do start searching the web for material about Ted Nelson, do not start with the Wired article about him from a decade or so ago. It is terrible. I quite like his YouTube videos, but they require to be familiar with his work (or at least his rhetoric style) already. I think "Geeks Bearing Gifts" is the best foray into Nelson's style - and it is short enough that one can read it quickly. Then, if you want to really get into the meat of his stuff, dive into his other books.