Usually a sports bra, and my favorite is Moving Comfort, but I'd like to try the Ibex next time I can afford it.

Sometimes I ride for a NON-workout - errands, toodling around with the kids, etc. - and for those times I just wear whatever it is I have on, which is usually an underwire, tee, and yoga capris. I don't change into a special outfit.... because it's easier just to hop on and go. But if I am going for a workout and plan on working up a sweat, I will make the effort to change into cycling clothes w/ a sports bra.