Real Academia - Diccionario de la Lengua Española
The Diccionario de la lengua española de la Real Academia Española or DRAE is the most authoritative dictionary of the Spanish language. With over 85,000 entries and almost 200,000 definitions, including many referring to terms from Latin America and the Philippines, the DRAE also gives indications of usage and historical significance. An essential online reference tool for anyone working with the Spanish language.
The Concise Oxford Spanish Dictionary 4th edition is highly recommended, but not available as an ebook for libraries. For sale on iTunes for $11.99; worthwhile especially if you're traveling or studying abroad. Completely updated to include the very latest vocabulary, this bilingual dictionary offers authoritative coverage. Brand-new features include in-text notes on life and culture in the Spanish-speaking world. 170,000 words and phrases; 240,000 translations. A print copy of this dictionary is on order for Olin Reference.
Oxford Reference Online - Pocket Oxford Spanish Dictionary Less comprehensive, but available online to the Cornell community.