Clues to code-breaking puzzles are frequently expressed as synonyms or related words. Also antonyms (words with opposite meanings) are sometimes used. With a thesaurus, you will find a list of all synonyms for a given word.
WordNet is a large lexical database of English, available for free. It is provided by Princeton University. It groups nouns, verbs, adjectives and adverbs into sets of cognitive synonyms. Therefore is resembles a thesaurus, but WordNet interlinks not only word forms, but specific senses of words. It also labels the semantic relations among words.
WordNet is available both for online browsing and downloadable database packages and source code.
See also: Code-Breaking overview | Anagram solver | Convert case | Crossword solver | Cryptic crosswords | Remove accents | Remove spaces | Reverse text | Text extractor | Upside down text | Word unscrambler