Zettel.io is a set of ways to organise and work with notes whether alone or in groups, whether in public or private and without caring what note taking tool is used to produce those notes.
It takes as its model the Zettelkasten, a cabinet of hooks upon which notes, strips of paper (zettel) could be hung in groups in relation to any mapping the researcher chose to have. The hooks in this case are the tags that notes acquire or are given and the association of notes together generates the insights.
Zettel.io is in alpha-- stage. This is more than a proof of concept but less than an operational and scalable product. It is being developed in public incrementally, sometimes very slowly sometimes not and without fanfare. But it is not secret and it is getting ever closer to making it to a beta release.