* Thesaurus.com
* Synonyms.com
* WordHippo
* TheFreeDictionary.com
* Print:
* Merriam-Webster's Thesaurus
* Oxford American Writer's Thesaurus
* Roget's International Thesaurus
* Longman Dictionary of Contemporary English
* Software:
* Microsoft Word
* Google Docs
* LibreOffice
* iBooks Author
* Mobile apps:
* Thesaurus.com
* Synonyms.com
* WordHippo
* TheFreeDictionary.com