Any music, really. If ever I need to relax I just slap on some Beatles, nice bubble bath with rubber duckies and I am in heaven. I don't listen to any kind of speed metal or bollocks like that so any music that I like tends to do the job.
I agree with Mercedes - pretty much any music that I enjoy calms me down. When I'm feeling angry or stressed out, I just pop the headphones in and turn the music up.
I usually lock myself in my bedroom, plug my headphones in, and just lay in my bed listening to music. The music I listen to depends on my mood, but I pretty much listen to anything so long as it's stuff that I really enjoy.
Any form of white noise, specifically the sounds of a fan. These silent Dyson fans were sent from the devil. A loud, whooshing fan is just so relaxing to me.
My creepy ventilation system. My grandma came and stayed for awhile and said that our vents sounded like aliens were invading and that she could hardly sleep. I personally like the sound they make ^^.