I just stumbled across this excellent, free, online resource: Wordreference.com, which searches across English, French, Italian, and Portuguese all once, and optionally from any of those languages to nearly any of the others. How incredibly useful!
It's very good with idiomatic expressions. For example, take a look at the entry for cabeza. You get a huge list of expressions using the word, with links to other entries in the dictionaries that contain it. You can hear the word spoken if you click on the icon for the sound file.
If you use Firefox you can also click a link near the bottom of the page to add a Wordreference search box to your toolbar.
There are forums to ask questions, too, and a list down the left sidebar of words nearby in the dictionary so you can browse.