An indispensable tool of the English language

It's a bit used in the Francophone world tool but a real reference in the English speaking world: the thesaurus. This is not a dictionary, it's better than a thesaurus is a collection of words in the English language in an "organized". How to find a word, how to find words of the same semantic field, how truly enrich their vocabulary? Using Roget's Thesaurus!