The Knight Library has many Spanish and bilingual dictionaries, including historical and topical ones, both in the Reference section on the first floor and in the stacks on the third floor. To browse the collection of Spanish dictionaries that you can borrow, go to the third floor and check the call numbers between PC 4625 and PC 4670, or do a subject search in the library catalog for "Spanish language dictionaries English". English-Spanish bilingual dictionaries are at PC 4640.
In addition to being a free online dictionary, Word Reference also has language forums where native speakers and language learners can discuss meanings of words in context of phrases or geography. For example, the metro in Spain is the subte in Buenos Aires, Argentina.
Another great site for looking up how to say a phrase is Linguee. You can search a whole phrase, and the site will search the internet to find comparable phrases on other sites. For example, the English idiom "pulling one's leg" means to tease or trick someone, but in Spanish the expression is "tomar el pelo" or pull a hair. Watch out for literal translations!
Do you know of another useful site? Please send your suggestions to Jeff Staiger, firstname.lastname@example.org.