When Harry, Ron, and Hermione are hiding from Voldemort's supporters in Deathly Hallows, they keep complaining that they have no food, and that Hermione sucks at cooking. Harry even bemoans the fact that, on the night they go on the run, Kreacher was making shepherd's pie that night for dinner. Why didn't Harry simply call Kreacher, as it's shown throughout the books that house elves can instantly Apparate to their masters wherever they are? Seems a waste to leave your house elf all alone when he could just travel with you.
This is answered in chapter 14 of HP and the Deathly Hallows, shortly after they've realised Grimmauld Place is now out of bounds (emphasis mine):
Harry wanted to believe that Kreacher had changed toward him in the past month, that he would be loyal now, but who knew what would happen? What if the Death Eaters tortured the elf? Sick images swarmed into Harry’s head and he tried to push these away too, for there was nothing he could do for Kreacher. He and Hermione had already decided against trying to summon him; what if someone from the Ministry came too? They could not count on elfish Apparition being free from the same flaw that had taken Yaxley to Grimmauld Place on the hem of Hermione's sleeve.
HP and the Deathly Hallows, chpater 14, The Thief
This is answered in the Deathly Hallows when Harry and Hermione agree calling him from a Death-Eater-infested Grimmauld Place is too dangerous - they don't know if the magic of the house-elves would prevent them from holding on and being dragged to their hiding place.