Encyclopedias & Dictionaries

LookWAYup
http://lookwayup.com/free/dictionary.htm
Correct your spelling, learn definitions, find similar words, and more.

RhymeZone: Rhyming Dictionary and Thesaurus
http://www.rhymezone.com/
Type in a word and get a list of rhyming words, as well as synonyms, antonyms, related words, quotes, and more.

Word Central
http://www.wordcentral.com/
An assortment of fun word-related activities, along with an online dictionary.