Without reading the other replies yet I'm gonna say a flare gun and a bottle of bourbon. Even if you're stranded you need to stay in the holiday spirit, that's why you need the flares. 
I have a rain jacket (the regular kind not a condom) with a hood, flashlight, basic toolkit (pliers, screwdrivers), blanket, and I have a weather radio somewhere in there. I keep forgetting to put another bottle of water in there. Gotta have a chaser.
I like to carry flares, mainly because I just like flares.
Besides, incompetent motorists will drive right into a car with its hazard lights on. Nobody ever hits a flare.
For the northeast winters I pack this into the trunk: For me: Army winter jacket, extra hat, gloves, socks, ski goggles, 2 MREs, handwarmer things, blanket, towels, 2 flashlights, firestarting stuff, candles, flares, knive, multitool, extra cash.
For the car: oil, brake fluid, anti-freeze, extra gal of gas, tow strap, shovel, hatchet, full toolbox, jumper cables, rope, ratchet straps, duct tape, giant zip ties
I do need to add a first aid kit, as I don't have one all the time. I'm sure there is some other junk in there as well. I usually end up using many of these items in non-emergency situations, so I have become known as prepared guy and can usually help someone out if needed.
Thermos full of ice tea, fishing pole, and a pair of flip flops.
Hey, you said emergency, right? 
Canned dog food. Most people will not snack on it, but it will save you from starving.
Tool box with a Torx set (it is a Jeep after all). extra engine belt and tool to change it.
First aid kit
Two spare tires.
Power inverter
fire extinguisher
shovel
Blanket
food, non perishable (in the cooler)
Bottled water
Candles (amazing how warm they can get the cabin of a car)
Matches and a grill lighter
Dry socks, extra gloves and hat.
Other stuff.
this is what we took to the Arctic circle.
pilotbraden wrote: Canned dog food. Most people will not snack on it, but it will save you from starving.
I mean sure... a few years after the collapse of civilization once people are used to wearing clothing woven from old money and Broker pelts, and you can't find a rusty old can of Chunky soup in the rubble of a grocery story anymore... I might bust open the dog food... but only after considering the effort of cooking and cleaning actual dogs first.
Get one of the aluminum floor jacks that HF always has on sale. They're cheap, and they're light, but they're floor jacks, so they should work awesome.
A cell phone?
I work/live in a major metropolitan area.
However, the summer tires on the MS3 will be interesting if we get any of our legendary sleet/snow this winter.
How about these guys and their truck? They seemed to have been prepared. Even brought their own antifreeze.
alex wrote: What I've heard from this thread so far: whisky, brandy, condoms, porn, .40 S&W, knives and a snowmobile. Best. Shopping list. Ever.
Gigity.
Joey
Flare gun, in case you need to kill a badguy in an epic way.
Joey
That's why you carry a Mosin it doubles as a flame thrower if someone is at close range.
Good thread..
I passed a bad accident the other day that had just happened. No paid responders on the scene, but many people had stopped to help. I contemplated stopping, but realized I didn't even have a 1st aid kit in the car. I may have the skills to help, but had nothing to help with, so I kept on motoring. The fact that about 6 cars had already stopped wanted me to get out of the way as well. Too Many cooks in the kitchen can be a bad thing.
I need to get on this though.. I think everyone needs at least a 1st aid kit and a fire extinguisher, then adjust per the season.
joey48442 wrote: Flare gun, in case you need to kill a badguy in an epic way.
I carry a banjo for exactly this purpose.
z31maniac wrote: A cell phone? I work/live in a major metropolitan area. However, the summer tires on the MS3 will be interesting if we get any of our legendary sleet/snow this winter.
Good luck with that lol, I attempted an MN winter on the stock potenzas on my 07...... that never happened again. Entirely too scary.
SVreX wrote: Thermos full of ice tea, fishing pole, and a pair of flip flops. Hey, you said emergency, right?
Ah, that'd be less funny if I didn't recently have to turn my Florida survival kit (sunglasses, board shorts, sunscreen) into a kit designed to keep me alive in this frozen tundra. blanket, beanie, gloves, hand warmers, etc.
My kit is a cell phone, the clothes I'm wearing, and a remote for my car.
My commute is within walking distance of either my place, my parents place, or my warehouse.
My van dies, I get out, call for a tow if needed, if tow time is long walk to nearest heated locale with boots on feet. No extra weight needed 
my commute is 2 miles tops, all "city" streets.
Come-along, tow strap, jumpers, 5L plastic fuel can, flashlight, insulated work gloves, spare windshield washer fluid, shovel, compact spare, tire plugs and wally world 12V compressor.
If its really cold out, or snowy, I'll throw in -20C sleeping bag and I always carry fresh water bottle. I also think that the candles are a really good idea...
At least none of you seem to have gotten into yourselves in this situation: http://www.theweathernetwork.com/news/storm_watch_stories3&stormfile=stranded_motorists_rescued_n_141210?ref=ccbox_weather_topstories
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