Sword of Aeons
At the end of the game, if you opt to kill your sister, watch he end credits in their entirety and you'll be able to continue playing armed with the awesome Sword of Aeons which kills most enemies with one swipe.

Skorn's Bow
If you make a series of sacrifices at the Temple of Skorn at midnight, you'll earn yourself the Skorns Bow. To get 'followers' to the temple easily, hire mercenaries to fight with you and then lead them there.