Dictionary of Spanish
Spanish/English dictionary

Lenguaje.com
The "Herramientos" section includes help with synonyms and antonyms, spelling, and conjugations

Diccionarion de la Real Academia Espanola
The most complete dictionary of the Spanish language, also look at Nuevo Tesoro Lexicográfico de la Real Academia Española.

Glossary of Spanish-English for Business
Translates business terms from Spanish into English and vice versa.

Diccionario Monolingüe Salamanca

Diccionarios de Sinónimos y Antónimos