Depends on your commute.
For me it is 14 rural miles at night.

A comfortable, lightly loaded backpack for windbreaker and wind pants, couple of granola bars ect.
Bike rack with trunk pack in case I need to pick up a few grocery items.
Cable and U locks. I leave these at work so I don't have to lug them around.
Fenders.
Basic tools.
Spare tube and patch kit.
Tire pump.
Very bright headlight (tactical flashlight on the handlebars in adjustable homemade mount) and taillight that uses the same batteries (AAA rechargable) .
3 spare AAA batteries (lithium primary).
Strap on LED headlight for backup light.
Waterbottle.
Cell phone.
No flames please but I always wear a helmet.