This is a collection of some useful synonym and antonym dictionaries.
Dictionary for synonyms, antonyms and definitions in English (AE as well as BE).
A collection of several thesaurus' editions for word lookup in British English.
This site provides a thesaurus in English.
The "Visual Thesaurus" provides an interactive map of familiar words of a given term.
This site offers an English thesaurus.
A collection of homonyms of the English language.
myenglishgrammar.com provides grammar lessons, as well as example lists of synonyms, antonyms and homonyms.