D! Links

Dictionaries
I turn to Word Reference constantly- it's great for finding the translation of basic terms as well as common expressions and generally has some useful examples. The forums are great when you don't see exactly what you're looking for and/or want clarification on a point. 

Word Reference (EN-IT) (IT-EN) (Dizionario italiano)

While Word Reference has a lot, there are some obscure or less-commonly used vocabulary that can often only be found on Italian-based sites. These sites usually offer more examples too.  I use these sites most often when I want just the Italian definition, and not necessarily the English translation.

Hoepli.it Dizionario (IT-EN) (Dizionario Italiano)
Corriere.it Dizionario (IT-EN) (Dizionario Italiano)
If you don't see what you're looking for on any of the above, it might not be a word that you need to know... but just in case, some other resources:

The Free Dictionary by Farlex (Dictionary)
Nice because it searches multiple languages at once so you don't have to switch back and forth between dictionaries.

Pronunciation
Forvo (Site in English with links to pronunciation of Italian words)
While many sites offer the phonetic transcription for each term, it can be difficult for people who haven't studied phonetics to really imagine how the word sounds.  This site offers audio files of several words, and often, more than one speaker so you get a feel for how the word can vary by region.

RhinoSpike
This site is great for seeing the pronunciation of entire phrases and sentences and not just individual words. Browse through others' audio requests and hear them recorded by native speakers- or if you're looking for something specific, make your own audio request and even specify which region or accent you prefer.  In my experience, users are quick to record your request- especially if you speak one of the languages they're studying and complete an audio request for them too.

Slang
L'Espresso (Slangopedia)
Especially fun for browsing slang terms, this site is updated by users who are constantly submitting new expressions so you know it's up-to-date and how Italians really speak.  (All terms and explanations in Italian)

See also: