“One day,” said Hermione, sounding thoroughly exasperated, “you’ll read Hogwarts, A History, and perhaps that will remind you that you can’t Apparate or Disapparate inside Hogwarts.”
“Well, he [Dumbledore] can’t have Disapparated!” cried Umbridge. “You can’t inside this school—”
Quietly, tentatively, Harry spoke into the darkness.
There was a very loud crack, and the sounds of scuffling and squeaks filled the silent room. Ron awoke with a yelp.
. . . Two house-elves were rolling around on the floor in the middle of the dormitory . . . .(HBP19)
As the war against Voldemort heats up, with Dementors on the loose (and breeding (HBP1)), giants stirring up trouble (HBP1), escaped Death Eaters using the Unforgivable Curses and, most of all, Voldemort himself having regained his full power, it’s going to be tough sledding for Wizardkind in general, and Harry, Ron, and Hermione in particular, to triumph. But while Voldemort maintains his hold on people and creatures through fear and intimidation, having no close friends of his own (HBP17), Harry has, over the years, gained a fairly impressive array of allies on his side through the power of friendship, loyalty, and love.
There are of course Ron, Hermione, and Ginny (not to mention the rest of the Weasleys), as well as Harry’s other friends at Hogwarts (for example, Neville and Luna). But even more, Harry has gained the affection and trust of Centaurs (Firenze), giants (Grawp) and half-giants (Hagrid and, probably, Madame Maxime), a phoenix, Thestrals, and a werewolf. But there is also a race of creatures, creatures who have no love for Voldemort (CS2), and one of whom has a huge amount of respect and admiration for Harry, who are powerfully magical in their own right, and could play a major role in the final book: the house-elves.
Consider the quotes at the beginning of this essay. As Hermione and Umbridge note, wizards and witches—even ones as powerful as Dumbledore—cannot Apparate and Disapparate inside Hogwarts (at least, not without Fawkes’s help). And yet Kreacher and Dobby both can not only do so, they can hear Harry whisper their names from several floors away and appear almost instantly. Indeed, it is worth noting that Dobby, for whom Harry did not call, heard him and came as well. (Although it is possible that Dobby appeared because he was fighting with Kreacher at the time (HBP19).) Dobby in his admiration for Harry seems to have set up some sort of magical alert that lets him know when Harry calls him—not a simple card or coin trick, for sure!
There are a number of other examples that show how powerful house-elves can be. Dobby levitates a cake without speaking a spell or using a wand (CS2). While an advanced wizard (post-O.W.L. level, HBP9) learns unvoiced spells, so far as we have seen, they always need a wand, yet Dobby levitates the cake without either. Dobby is also seen blasting Lucius Malfoy off his feet (with something that appears to be the Expelliarmus hex), again without a wand or audible incantation (CS18). The meals and feasts at Hogwarts are presumably prepared and served using house-elf magic. And finally, it is clear that Wizardkind is very nervous about the power of house-elves, otherwise why would they pass such a stringent law and make the punishment so severe for an elf to possess and use a wand (GF9)?
There are of course powerful charms working against the house-elves. An elf in service finds it extremely difficult to so much as speak out against their master, and outright disobedience is apparently completely impossible (HBP3). Elves also appear (in general) to prefer a life of service to being free (GF21)—Dobby being a notable exception. Even so, with elves displaying the level of magic of an advanced witch or wizard without a wand, the Ministry and the Wizengamot must have felt it critical to keep anything that would magnify their powers out of their hands.
So if and when Voldemort arrays his giants, trolls, dragons, and who knows what else—perhaps Blast-Ended Skrewts?—against Harry and the other folks resisting him, it’s good to know that Harry and the rest will hopefully not be facing them alone, but perhaps have the powerful magic of house-elves (enhanced by special wands created by a liberated Ollivander?) on their side, fighting not out of fear, but rather love and respect for Harry and the others. A powerful force indeed.