I've really been impressed with Ultralingua. Although it is not free, their iPhone app for French is terrific, especially for verbs where it gives the conjugation in every tense and mood for every person, single and plural. The dictionary is excellent because it gives you many phrases and expressions in which the defined word is used. They also have a Mac version that I have on my MBP. Their content for French/English is based on a well-regarded and well-known bilingual dictionary, and I wouldn't be surprised if they didn't take a similar approach to Spanish.
I know they do offer a Spanish version (as well as one for many other languages), and based on my experience with the company, I'd recommend you at least check out the reviews.
Buena suerte!