There's lots of helpful information locked away in databases that can never be indexed by search engines. The services below let you search this "invisible web" or "deep web."
From Intelliseek, the makers of the BullsEye meta search software, InvisibleWeb.com catalogs searchable resources across the web. Also used for the Lycos Invisible Web service, below.
Lycos Invisible Web Catalog
Produced in partnership with Intelliseek, this search engine gives you access to all sorts of online databases.
A large collection of specialized search tools that often contain info that is hidden to search engines. Lovingly compiled by search expert Gary Price.
Comprehensive guide to searchable databases. Browse or search through listings.
At SES London (9-11 Feb) you'll get an overview of the latest tools, tips, and tactics in Paid, Owned, Earned, Integrated Media and Business Intelligence to streamline your marketing campaigns in 2015. Register by 31 October to take advantage of Early Bird Rates.