Learning Spanish isn't easy, so it's a really good idea to have a list of dictionaries that you can use to make sure the word you want to use is actually the word you end up using.
The Real Academia Española is the official arbiter of Spanish, and on their website it is possible to lookup a Spanish word to get the official meaning, in Spanish of course.
Business Spanish If you're in business, and you need to communicate with Spanish business people, then this dictionary is the one for you.
Spanish slang Slang and rude words from all over the Spanish speaking world.
Universidad de Oviedo English/Spanish dictionary, dictionary of synonyms, translator, verb conjugator from the University of Oviedo.
Spanish CALL project
Grammar rules and interactive exercises, tools and links to other sites
Diccionarios.com Several languages on one site, although you'll most likely only be interested in the Spanish and English translator, but this site also includes a verb conjugator.
Netaurus A site to learn about Spanish culture throughout Spain and the Americas. Learn about cuisine, music, culture, even siesta and festivals
Learn Spanish online is a site that actually gives you exercises, interactively, and from beginner to advanced levels.