This one is called Farewell to Dobby. By Alexandra Desplat and plays in DH Part 1.

I feel like paying tribute to Dobby whenever I hear this.

We all remember that cute little elf who we first saw on Harry's bed in the second book/movie and got him into trouble. Back then, he was the Malfoys' elf. And now you see how far he's gone. He sacrificed himself to save "Harry... Potter..."

Farewell to Dobby, a free elf.