Carrot2 is an Open Source Search Results Clustering Engine. It can automatically organize 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 is implemented in Java, but a native C# / .NET API is also available. Other non-Java platforms, such as PHP or Ruby, can call Carrot2 clustering through its REST interface.