Some of the dictionaries available under the category "Thesauri":
Chambers ThesaurusRoget's II ThesaurusOxford Thesaurus of EnglishMoby ThesaurusUse the Right Word
Chambers Thesaurus

Merriam-Webster's Thesaurus

Roget's Thesaurus

Oxford Thesaurus

Oxford Learner's Thesaurus

Moby Thesaurus

Use the Right Word