For engineers, scientists and other technical professionals, GlobalSpec has completely revolutionized the way engineering work is done. GlobalSpec was founded in 1996 for engineers by engineers and leveraged its domain expertise and deep understanding of the online behavior and complex needs of the engineering and technical community to build The Engineering Search Engine. To achieve this, GlobalSpec has crawled, filtered and indexed the World Wide Web and focused solely on engineering-related information. Included in The Engineering Search Engine is access to the hidden Web - content protected by firewalls, requiring registration or fees to access. In other words, content not readily available to general, mass-market search engines.
Link to resource: GlobalSpec: The Engineering Search Engine