Find Mii, known as StreetPass Quest in the PAL region, is a game that comes with the Nintendo 3DS. It can be played by opening StreetPass Plaza from the 3DS home screen and choosing the treasure chest icon to the right of your StreetPass Mii.
The December 6, 2011 update added Find Mii II (StreetPass Quest II), which can be played after you collect all of the hats in the first game.