I use my iPhone when I run, and I LOVE the pandora app for running. I usually listen to either the Fun. Or Rise Against stations. I like the mix I get there. I can't run with a set playlist.
I'm currently into Selena Gomez tracks (facepalm) nowadays and added them to my running playlist. When I'm doing my marathon training program, I always listen to rock music like Blink 182, Korn, Limp Bizkit, etc. because it keeps my energy up - the angst and the strong drum beats keep me motivated as well.