At the wiki entry for Arya the following list is given:
Dead
- King Joffrey. Posioned by Olenna Tyrell.
- Ser Amory. Eaten by Vargo Hoat's bear.
- The Tickler. Arya killed him herself at the Crossroads Inn.
- Polliver. Killed by The Hound at the Crossroads Inn.
- Ser Gregor. Died in extreme pain due to the thickened manticore venom that coated Prince Oberyn's spear.
- Raff the Sweetling. Killed by Arya under her guise of "Mercy" at the Gate, a mummers playhall in Braavos.
Alive
- Ilyn Payne
- Meryn Trant
- Queen Cersei
- Dunsen
Removed
- The Hound
Reanimated:
- Ser Gregor a.k.a The Mountain (He was reanimated by the alchemist)
Source: http://awoiaf.westeros.org/index.php/Arya_Stark#Arya_and_Death
One might add that "the Hound" is sort of dead, at least according to the story of the elder brother of Quiet Isle.
Another interesting tidbit is the prophecy given to Cersei by Maggy the Frog, which implies that Cersei will die by the hands of "the valonqar", which is high valyrian for "little brother". Cersei always thought it was Tyrion, but perhaps it could also refer to a certain "little sister"?
