So far, my must haves have been:

Helmet
Gloves
iPod (I listen to it in one ear, leaving the other open to hear)
Cellphone
Water
License & debit card
Portable hex wrenches thingermabob
Cyclocomputer
Windbreaker than I can stuff in my handbar bag
Keys (if I am driving to a trail first)
Bike lock (around my seatpost)
Cheap HD yellow safety glasses (indigoiis! Probably the same ones )

My RoadID just came in the mail today, so that will go with me, too. I also plan on getting an extra tube to keep in my bag, as well as the tire levers, a small pump, and a multitool (thanks to your suggestions).