I use the Mac (and iPhone) Dictionary from Ultralingua. I swear by it. It'll let you search for words even if your search term is a conjugated verb. It also gives you the verb conjugations.
(I'm using the Collins Pro version, not the cheaper one, which, at least for Italian, was inadequate)
http://www.ultralingua.com/
It's not a translator, but it will provide you with more than a paper dictionary. Lots of examples of how to use each word in context etc. If you're reading books, you don't want to use a translator anyway (mostly), you want to use a dictionary.
These online translators are only available online party because of the way they're implemented. They're constantly learning (at least the good ones like Google and Babelfish are) based on the inputs and people making corrections.