I have mentioned the invisible web (collections of databases that search engines do not index) many times before. As resources go, databases are possibly the best. Organized, categorized and often verified, they provide usable and timely information to the academic or journalistic researcher.
A truly excellent guide to the invisible web can be found at the Online Education Database.
Sections on its´background and growth, specific search tools and services make this a one stop shop for" hidden web" information.
While you are at it have a look at their Research Beyond Google: 119 Authoritative, Invisible, and Comprehensive Resources