Carrot2 is an open source software for finding thematic clusters in groups of documents…

“It can automatically organize [and label] small collections of documents, e.g. search results, into thematic categories. Apart from two specialized document clustering algorithms, Carrot2 offers ready-to-use components for fetching search results from various sources including YahooAPI, GoogleAPI, Bing API, eTools Meta Search, Lucene, SOLR, Google Desktop and more.”

“Carrot2 came about as a framework for building search-results clustering engines but its algorithms should successfully cluster up to about a thousand text documents, a few paragraphs each”