Thesauruses

Monolingual, English

Merriam-Webster Online

Thesaurus.com

yourdictionary.com

Roget's Thesauri

Encarta Thesaurus

Walters English Thesaurus

RhymeZone - rhymes, synonyms, definitions

Multilingual, including English

GEMET - GEneral Multilingual Environmental Thesaurus

Monolingual, Norwegian

Norsk miljøtesaurus