I discovered it some three years ago (I think) and, together with WordReference.com and Rae.es, is one of the best ones for me.
Oh, sorry! This is the link to this online dictionary! The American HeritageŽ Spanish Dictionary
Apart from offering translations of words for both languages, it also includes an icon where you can click and listen to the pronounciation of a concrete word.
So, type any word on the "Search Spanish Dictionary" box and you won't just find its translation, but also how it sounds and... something I find quite interesting and useful: a list of idioms containing that word. This is the only online dictionary, apart from WordReference.com, (as far as I know) that includes lists of idioms.
More features: On the main page, if you scroll down, you'll find information links (in both languages) on how to use this dictionary, some notes on grammar, a Spanish verb table, Spanish orthography rules and more.
So, bookmark this dictionary as I think it can be very useful.
Hope this helps!