Real Academia de la Lengua Española. Everything on the Spanish language. You can consult doubts with this very effective service.

Diccionarios.com. Five dictionaries that can consult online.

Diccionarios de elmundo.es. It chooses the dictionary: Spanish Language and its synonymous of antonyms.

Foreignword.com. It searches 265 different dictionaries (bilingual and monolingües) available in Internet.

La Lengua Española. A very inclusive webpage on the Spanish language.

AllWords Dictionary - English dictionary with multi-lingual search