iTools. Online dictionary (Webster’s), rhyming dictionary, thesaurus, maps, phone directories. etc.
http://www.itools.com/research-it/research-it.html
WordNet. Princeton University’s online thesaurus that groups nouns, verbs, adjectives, and adverbs in related categories.
http://www.cogsci.princeton.edu/~wn/
Onelook. Online dictionary, impressive in scope. Contains almost 3 million words.
RhymeZone. A rhyming dictionary online and a whole lot more. Type in a word and receive virtually very rhyming word in English. Very extensive listings, however, including arcane words. Subdivides results by number of syllables. Also provides (on request) definitions, synonyms, and antonyms. It will even locate the word in Shakespeare and other sources.
WriteExpress Online Rhyming Dictionary. Lets you choose the type of rhyme you want (end rhymes, last syllable rhymes, double rhymes, beginning rhymes, even first-syllable rhymes).
http://www.writeexpress.com/online.html
Readability Sites. Just in case you lost your print copies, two readability formulas are online: the Fry and the SMOG – directions, tables, graphs, everything you need. Just to keep such formulas in proper perspective, however, remember that SMOG stands for "Some Measure of Gobbledygook"!
http://school.discovery.com/schrockguide/fry/fry.html
http://www.cdc.gov/od/ads/smog.htm
Merriam-Webster Online Dictionary. Offers quick look-ups plus a thesaurus, word games, word of the day, more.
http://www.m-w.com/dictionary.htm
Encyclopedia Britannica. The entire contents online with a convenient search engine and many related links.
Library of Congress. Home page of the most extensive information system in the world (except for the Internet!). Sections for parents and kids. Maps available.