Words For That - the reverse-Urban Dictionary of the internet
FoxType Thesaurus - Visualize similar words for _____
Lingvist - Lingvist helps you take your foreign language skills to the next level with a broad selection of exercises and lessons.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley
Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.
Writefull Thesaurus - A Google Docs add-on that uses AI to give you synonyms