Without a doubt, it's my iPod.

I can't do cardio without music. Period.
Close behind is the pad that I put on the barbell before I do squats and lunges. I'm such a wimp that the weighted bar kills my back otherwise. The one the gym has stinks, so I bought my own.
Love my new running shoes -- Mizunos Wave Rider 7. Never heard of them before but they were recommended at the store and fit my feet very well.