Bare necessities are:
Gloves
helmet
glasses
cycling-specific shorts/pants
extra inner tube
pump with tire lever
water bottle and cage (if not, a camel-back or equivalent)
ID with some cash

Useful extras:
Multi-tool with hex sizes you need
chain tool with extra chain links
bandages or first aid kit
speedometer/heart-rate monitor (if you're serious about training)
pepper spray
knife
gun