Encyclopedias

Wikipedia  (The free encyclopedia that anyone can edit) 

Encyclopedia Britannica (Explore the updated online encyclopedia from Encyclopaedia Britannica with hundreds of thousands of articles, biographies, videos, images, and web sites)

Encyclopedia (Encyclopedia.com has more than 100 trusted sources, including encyclopedias, dictionaries, and thesauruses with facts, definitions, biographies, synonyms, pronunciation keys, word origins, and abbreviations)
 
Reference (A free online encyclopedia and information reference. Research articles from Columbia Encyclopedia and Encyclopedia Britannica)

Thesaurus (the largest and most trusted free online thesaurus brought to you by Dictionary.com. Quickly find synonyms and antonyms) 

The Canadian Encyclopedia (Covers a wide range of topics such as music, sports, birds, and the railway. Includes interactive maps, graphs, games, and quizzes) 

New World Encyclopedia (Organizing knowlefge for happiness, prosperity and world peace)  

Information Please (Part of the Family Education Network) 

Fact Monster (Fact Monster is a free reference site for students, teachers, and parents. Get homework help and find facts on thousands of subjects, including sports) 

Answers (Free Site - Does not have popups but does have banners. Entries are separated into two groups: people, and all other subjects)

The Free Dictionary (Site does not have any advertisements, but some content only available with a subscription)