“Knoten” is German for a node in a graph.
With Knoten, you can download data from the the Web and integrate it with your desktop. You can download things like your del.icio.us tags (soon ;-), RDF files, or Topic Maps (soon ;-). Each tag, resource or topic will become an object on your desktop – it will be indexed by Spotlight, and you can connect it to other objects (e.g. with Kante), like the contacts in your address book or events in your calendar.
You could, for example, download an ontology of countries and link your contacts to the countries they live in, or your events to the countries where they take place. In other words, you will tag files and objects on your desktop, but instead of tagging them with plain text strings, you will tag them with URIs which are used by many other people as well.
There is a short screencast about Knoten. It is currently without comment, but there is a script that shows you what the comment would be, if it were there.
Always a good thing to do: read the readme file!