What does Draco Malfoy use to get the Death Eaters into Hogwarts?

He uses a wardrobe from the Room of Requirement.

ok, to clarify the previous responses, he uses a thing called a Vanishing Cabinet that can be found in the room of requirement. the other vanishing cabinet is in Borgin and Burkes, the dark arts store in Knockturn Alley. the idea is that whatever is put into the cabinet from one location will appear in the cabinet at the other location. So all the death eaters got into the cabinet in Borgin and Burkes, and then were transported to Hogwarts.