04: Britannica Online, http://www.eb.com

It is the site of the Encyclopedia Britannica Online. It is one of the most important sources of encyclopedic knowledge of the world and perhaps the most important of the Web. They said about themselves:

The Encyclopædia Britannica is the world's most comprehensive reference product, a distinction it has held since its first publication in 1768.

EB Online includes the complete encyclopedia, as well as Merriam-Webster's Collegiate Dictionary and the Britannica Book of the Year.   You can also use EB Online to search an Internet directory that includes more than 130,000 links to Web sites selected, rated, and reviewed by Britannica editors.

Through this service, you can find more than 72,000 articles, updated and revised by EB editors and contributors; over 10,000 illustrations, including photographs, drawings, maps, and flags; and more than 75,000 definitions--including pronunciation guides and word histories--from Merriam-Webster's Collegiate Dictionary.

Important: This service has to be paid 5 dollars per month but you may get a 30 days free permission to check the whole site. However they offer almost the same service absolutely free in its associated site: http://www.britannica.com . We suggest you to register for the 30 days free period in the Encyclopedia to evaluate if the difference deserves the payment for your needs.