"'To Ronald Bilius Weasley, I leave my Deluminator, in the hope that he will remember me when he uses it'… "
Ron was given a Deluminator in Dumbledore's will. The trio used it when they were attacked by Death Eaters Antonin Dolohov and Thorfinn Rowle at Tottenham Court Road. Ron put the lights out while Hermione wiped the Death Eaters' memories, and he and Harry cleaned up the cafe. They used it again in the dungeon of Malfoy Manor as part of their escape.
Most importantly, Ron used the Deluminator to find Harry and Hermione after temporarily abandoning them in the winter. He instantly regretted his decision to leave, which was influenced by the negative energy within Salazar Slytherin's Locket, but was unable to find them until the sound of Hermione saying his name came from the Deluminator, which began to emit light which led him back to his friends. Ron saved Harry's life and destroyed the locket Horcrux. While Ron was glum about Dumbledore's apparent prediction that he would leave his friends, Harry pointed out that it meant Dumbledore knew he would always want to come back.