I don't visit Clusty much but as Google was erroring, I tried Clusty and lo and behold, they have this linked on the main page:
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Although clustering reveals the major topics in the top 200, 500, or more search results, there are always more topics than can be shown, without overloading the user with a very long list. There hasn’t been any better approach, until now.
With a single click, remix clustering answers the question: What other, subtler topics are there? It works by clustering again the same search results, but with an added input: ignore the topics that the user just saw. Typically, the user will then see new major topics that didn’t quite make the final cut at the last round, but may still be interesting.
Remix clustering was introduced in Vivisimo Velocity 6.0, our enterprise search platform which also introduced other user-experience capabilities.

