Links to Dictionaries / Resources
Medical Spanish - Spanish for healthcare: dictionaries and resources
Yahoo! Education American Heritage Spanish Dictionary - This site also has numbers, measures, verbs, grammar, and voice pronunciation of each word.
Real Academia Española - This is the definitive dictionary of the Spanish language, and is completely in Spanish. The site also includes grammar explanations meant for high-level students of the language.
WordReference.com - Large dictionary with forum discussions about expressions, etc.
Verbix - Extensive verb conjugator.