AltaVista had a Java applet that would visualize the "clusters" that a search produced. You could then click on a "cluster" in order to exclude all the irrelevant ones and the search results would update.

For example: Searching on "python" would give you two obvious clusters one for "reptiles" and one for "programming languages". Clicking on the appropriate cluster would screen out all the irrelevant ones.

This is a feature still unmatched by any search engine today.

I've never seen this feature despite using a lot altavista back then. What a pity