I spend a lot of time in my car for work (usually sitting in LA traffic) and upon arrival in Cali, quickly determined that an emergency kit was a necessity. If you live in an area that is prone to natural disasters, extreme weather patterns, or ever might be involved in a traffic accident ( Read: everyone), I'd recommend outfitting your car as well.
Here are the items in my kit:
A case with handles.
A case with handles.
A first aid kit, flashlight (with batteries not stored in the flashlight), tire leak spray, water, sturdy gloves, rope, jumper cables, an all weather poncho, a safety vest and red towels (don't use white).
Optional items include duct tape, a tire pump, oil, assorted tools, and WD-40. I also found an old ice scraper in ours which was funny since the only time I've needed one was last weekend in Yosemite and couldn't find one ...we used a credit card instead.