SpriteMaskFilter
to be exact. Works basically like this:
var mysprite = new PIXI.Sprite(),
mymask = new PIXI.Sprite();
mysprite.filters = [new PIXI.SpriteMaskFilter(mymask)];
var mysprite = new PIXI.Sprite(),
mymask = new PIXI.Sprite();
mysprite.mask = mymask