Carrot2 is an Open Source Search Results Clustering Engine. It can automatically organize small collections of documents (search results but not only) into thematic categories.
Search results clustered with Carrot2 (live demo)
Apart from two specialized document clustering algorithms, Carrot2offers ready-to-use components for fetching search results from various sources including GoogleAPI, Bing API, eTools Meta Search, Lucene, SOLR, 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. The downloads page will help you to choose the right package. For consulting services, installation, maintenance and text mining expertise, contact Carrot Search -- the Carrot2 spin-off company.