For more than 10 years this website provides comprehensive and easy-to-use thesaurus resource. It was created and launched in 2011 as a side project, but later it's grown into a self-sufficient website.
In general, it represents a common synonyms/antonyms search and optional visual representation of words connections. Additionally it shows more common words in the search results (aka "hypernyms"). For example, for the "car" synonyms search you will receive more general "vehicle" in the output.
Under the hood, EnglishThesaurus.net uses widely known Wordnet® lexical database.