Ciao - The informal word meaning hello or good bye depending on the situation. It is equivalent to saying hey or bye-bye.
Salve - The formal version of Ciao.
Pronto - Used as a greeting over phones.
Arrivederci - Means Goodbye. "a domani arrivederci" means see you tomorrow.
Buon Giorno - Literally means Good Day, but is used to mean both Good Morning and Good Day.
Buon Pomeriggio - Means Good Afternoon.
Buona Sera - Means Good Evening.
Buona Nocte - Means Good Night.
Come Stai - Informal and common way to say How Are You.
Come Sta - Formal and less common way to say How Are You.
Piacere - Common greeting meaning Pleased To Meet You, and Piacere may be used in reply meaning Pleased To Meet You As Well.
Benvenuto - Means Welcome, and is used often if the situation permits.