I have prepared three thesauri that I use most often. All of them are free and can be used any time you have access to the Internet:
1) Thesaurus.com – a good source of synonyms and antonyms.
2) Cambridge – has entries for both British English and American English varieties.
3) Merriam-Webster – visual, based on pictures – can be used by/for learners of English
Feel free to add any thesauri to this list!