Carrot2 is an Open Source Search Results Clustering Engine. It can automatically organize (cluster) search results into thematic categories.
As quoted from the website, Carrot2 provides an architecture for acquiring search results from various sources (YahooAPI, GoogleAPI, MSN Search API, eTools Meta Search, Alexa Web Search, PubMed, OpenSearch, Lucene index, SOLR), clustering the results and visualising the clusters. Currently, 5 clustering algorithms are available that are suitable for different kinds of document clustering tasks.
It has a search clustering plugin for Nutch so that you can integrate searching, crawling and clustering in one place.