There are plenty of dictionaries available online, but these are a few that I find particularly helpful:

WordReference has a very good multilanguage dictionary (Italian-English or English-Italian). You can also install a browser extension that allows you to search WordReference for a word from the search toolbar in your internet browser.
The normal Google search engine has a hidden function that allows you to search for definitions of any word by typing define before the word. For example, to define the word "funnel", you type "define funnel". Google suggests definitions from a variety of online dictionaries.
Like the Google define function, Dictionary.com searches multiple online dictionaries. Try it here:

Word a Day
These aren't dictionaries, but they're a fun way to learn new vocabulary. You can subscribe to have them sent to your email address, as an RSS feed, or just check them out here every once in a while. Warning: the OED (Oxford English Dictionary) sometimes includes Old English contexts that you will not understand. No one does.

For fun:
Watch this!

Creative Commons Attribution 3.0 License This site is the work of L. Hornbake, with participation by students.