The US Department of Homeland Security recommends every home have, at the bare minimum, these supplies. The reference to a period of three days can be altered depending on what kind of emergency you are anticipating:
Water (1 gallon per person per day)
Nonperishable food for three days
Can opener
Battery-powered weather radio with tone alert
Batteries
Flashlight
First-aid kit
Whistle to signal for help
Plastic sheeting for shelter, plus garbage bags
Duct tape
Dust masks
Moist towelettes
Wrench or pliers to turn off utilities
Local maps
Cellphone and chargers