Unlike traditional search engines, which crawl the Web gathering Web pages, Semantic Web search engines index RDF data stored on the Web and provide an interface to search through the crawled data. Below is a list of Semantic Web search engines that are currently under development.

Semantic Web Search Engine (SWSE)

SWSE is a search engine for the RDF Web on the Web, and provides the equivalent services a search engine currently provides for the HTML Web. The system explores and indexes the Semantic Web and provides an easy-to-use interface through which users can find the information they are looking for. Because of the inherent semantics of RDF and other Semantic Web languages, the search and information retrieval capabilities of SWSE are potentially much more powerful than those of current search engines. SWSE indexes RDF data from many sources, including OWL, RDF and RSS files. RSS2 is converted to RDF and they will be adding GRDDL sources soon. Developed by DERI Ireland.

Sindice

Sindice is a lookup index for Semantic Web documents built on data intensive cluster computing techniques. Sindice indexes the Semantic Web and can tell you which sources mention a resource URI, IFP, or keyword, but it does not answer triple queries. Sindice currently indexes over 20 million RDF documents. Developed by DERI Ireland.

Watson

Allows you to search through ontologies and semantic documents using keywords. At the moment, you can enter a set of keywords (e.g. "cat dog old_lady"), and obtain a list of URIs of semantic documents in which the keywords appear as identifiers or in literals of classes, properties, and individuals. You can also use wildcards in the keywords (e.g., "ca? dog*"). Developed by KMi, UK.

Yahoo! Microsearch

Microsearch is Yahoo!'s stab at Semantic Web search and provides a richer search experience by combining traditional search results with metadata extracted from Web pages. Indexes RDF, RDFa and Microformats crawled from the Web. Microsearch will soon be adding support for GRDDL.

Falcons

Falcons is a keyword-based search engine for the Semantic Web, equipped with browsing capability. Falcons provides keyword-based search for URIs identifying objects and concepts (classes and properties) on the Semantic Web. Falcons also provides a summarization for each entity (object, class, property) for rapid understanding. Falcons currently indexes 7 million RDF documents and allows you to search through 34,566,728 objects. Developed by IWS China.

Swoogle

Searches through over 10,000 ontologies. 2.3 million RDF documents indexed, currently including those written in RDF/XML, N-Triples, N3(RDF) and some documents that embed RDF/XML fragments. Currently, it allows you to search through ontologies, instance data, and terms (i.e., URIs that have been defined as classes and properties). Not only that, it provides metadata for Semantic Web documents and supports browsing the Semantic Web. Swoogle also archives different versions of Semantic Web documents. Developed by the Ebiquity Group of UMBC.

Semantic Web Search

Powered by RDF Gateway, Intellidimension's proprietary platform for Semantic Web applications and agents. Developed by Intellidimension Inc.

Zitgist Search

The Zitgist Query Service simplifies the Semantic Data Web Query construction process with an end-user friendly interface. The user need not conceive of all relevant characteristics - appropriate options are presented based on the current shape of the query. Search results are displayed through an interface that enables further discovery of additional related data, information, and knowledge. Users describe characteristics of their search target, instead of relying entirely on content keywords.