One of the most respected dictionaries of the English language, Merriam Webster offers completely free use of their site which includes definitions, pronunciation guides, usage guides, etymology, and synonyms.
This online thesaurus gives the user pronunciation guides, synonyms, antonyms, guides to informal and slang usage, and visual thesaurus word webs.
This rhyming dictionary divides relevant rhyming words by syllables. it also offers near rhymes (or "slant rhymes"), definitions, homophones, and consonant rhymes.
This etymological dictionary gives the detailed history of words including recorded usages, language family, and evolution.
This anthroponomastics website teaches patrons the origins of names. It sorts names by alphabet, country of origin, subject, meaning, gender, and has a search function. Unlike most baby name websites, it does not sugar coat the meaning and origins of names.
Reference, Trivia, and Homework Help
Discovery Education offers users free resources including text books, articles, games, homework help, and math tutorials.
This Library of Congress virtual reference shelf is sorted by subject, type of reference, and reference for parents and children.
Scholarpedia operates much the same as Wikipedia - it is all volunteer, user uploaded information. However, all of the articles are peer-reviewed before publishing.
Infoplease is a simple reference source somewhat akin to an almanac, filled with facts and trivia and searchable by subject. It also offers students homework help and provides daily trivia.
American Fact Finder offers the user innumerable facts garnered from census data about states, regions, cities, counties, communities and demographics.