This past week iTunes had a free song by James Blunt. As I went to download it, I recognized the voice, and that I'd heard another song by him that I'd liked. Looking in the iTunes 'popular download' part I saw the song listed: You're Beautiful and I was just about to buy it when I had a brain wave. Did I already buy this?
I looked in the Purchased Music folder of iTunes, and sure enough, there it was!