Jacqui Robbin's blog, Jacqui's Room, has a great series, No, Seriously. Just Try It. Today's challenge (Magic Objects) is involves the one object your protagonist always has with them.
My protagonist always has her cell phone with her. Perhaps you think that's mundane, but after all she is a normal teenager. Her cell allows her to communicate with her friends and even fantasize about contacting the guy of her dreams. Her cell also works as a music player, like an iPod. My younger son's phone is supposed to accept music downloads, but we could never make that function work. My protagonist is smarter than we are, certainly more electronically adept.
Anyone who has read Harry Potter and the Deathly Hallows knows that magical objects often have a dark side. The downside to Lia's cell is the antagonist can also call her. If you don't think that's a problem, you haven't met my antagonist.
While you're visiting Jacqui's Room, also check out the last No, Seriously, Just Try It.