Writefull Thesaurus - A Google Docs add-on that uses AI to give you synonyms

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.

Dictionary Anywhere - Dictionary Anywhere is an open-source and easy-to-use browser extension for Mozilla Firefox and Google Chrome that allows you to check definitions easily while browsing the web.

Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...

Silicon Valley Dictionary - Urban dictionary meets Silicon Valley

WordNet - WordNet® is a large lexical database of English.