What is Reversible?
Interesting concept, but as you can see in the about page, there are a lot of unanswered questions. Anytime you allow users to do whatever they want, you introduce crazy amounts of variability into the system, something that is hard to get back out. Very neat, and something to check out.
Oh, yeah, you want to know what it is: It takes referrers and categorizes them. You can create directories by creating a link, the site then takes that link and trates it like a category, showing people who have used the link to get there in the category. Weird, eh?