Draco Malfoy
In "Harry Potter and the Half-Blood Prince", Draco Malfoy uses a pair of enchanted coins to bypass the communication limits imposed on Hogwarts, thus managing to keep in contact with Madam Rosmerta, whom he had placed under the Imperius Curse. Draco reveals he got the idea from Hermione's DA coins, which were themselves inspired by Lord Voldemort's use of the Dark Mark to communicate with his Death Eaters.