Dictionaries
WordReference.com: The most comprehensive and nuanced Italian-English dictionary on the Web. Do not trust Google Translate! Instead, use WordReference and explore the multitude of definitions and subtleties of the Italian language.
Treccani.it: One of the best Italian dictionaries available for free online, and an especially useful tool when working with older Italian words and phrases that are difficult to find in contemporary dictionaries.
Grammar
Italian Verbs: A free Italian verb conjugator, containing more than 12,000 verbs
La Grammatica Italiana: Professor Christina Mazzoni (University of Vermont) created this extensive grammar website, which includes online exercises and helpful tips for retaining Italian grammar.
Pronunciation
Forvo: An online pronunciation dictionary, including over 85,000 words and phrases. Each word or phrase was recorded by a native Italian speaker, whose geographical location can be viewed along with his or her recording.
VOCABOLaudio: More than 16,000 high-quality audio files of Italian pronunciation, recorded by native speakers. Also available as an iPhone app!