WordNet - WordNet® is a large lexical database of English.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley
WordWeb - One-click lookup in any almost any Windows program; Hundreds of thousands of definitions and synonyms; The latest international English words; Works offline, or reference to Wikipedia and web references.
FoxType Thesaurus - Visualize similar words for _____
Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...
Writefull Thesaurus - A Google Docs add-on that uses AI to give you synonyms