Search the Diccionario de la lengua española, Diccionario panhispánico de dudas and more at the website of the primary organization responsible for the lexicography of the Spanish language.
Duolingo
Duolingo is a free online language learning platform that allows language learners from a wide array of language backgrounds to learn how to speak other languages, such as French and Spanish.
WordReference
English-language and language-English dictionaries.