My dog, of course.
(If I could only grab one thing he would be it) Other than that, my purse (wallet, keys, checkbook, work ID, bus/train pass, phone, Ipod etc are generally all in there), my Oscar the Grouch puppet which I've had since I was a baby, my photo box, glasses & contact case, my computer, my jewelry case (with all the stuff in it), my passport, as many books as I could stuff into a bag...heck, given enough warning, everything I could fit into my car.