2012 Update

Great thread! I have to make room for a baby stroller in the back of my Jeep, so all of this needs to be kept in a large rubbermaid bin in the back, along with the glove box, console and under the seats. Last year I was driving a little Honda Civic and had a similar load but with much more in terms of winter clothes. (It's still early in the season. I'll add snow pants for everyone when we go on road trips.)

From memory:

EDC backpack
Personal BOB
My son's daypack/bob
FAK

In rubbermaid bin in rear/under seats/in glove box/console:

Spare Clothes:
blaze orange thinsulate hat
reflective vest
extra fleece jackets
bright yellow rain slicker
extra insulated boots
thinsulte tappers hats
wool or thinsulate mittens
insulated mechanic's gloves

Shelter:
wool blanket(s)
winter sleeping bag (s)
thermarest
tarp
paracord
small 2 man tent

Tools:
emergency only cel phone
extra sunglasses
cel phone chargers (mine, hubby's and emerg phone's)
tool kit (ratchet set, screw drivers, wrenches, etc.)
LED Flashlights (1 X DD, 1 x AAA, lighter outlet plug-in)
extra batteries
multi-tool
bic
snap lights
pens
notebook
owner's manual for Jeep
jumper cables
tire iron
extra fuses
oil pressure gauge
air compressor
tow rope
thule straps
large coffee can
hobo stove
esbit tabs
fatwood
tea lights
newspaper
bic
strike anywhere matches
cotton balls
chem hand warmers
plastic garbage bags
TP
shop towels
rags
road flares
shovel
ax
folding saw
ice scraper
kitty litter or road salt
2 golf umbrellas
fire extinguisher
road maps
inflated (to pressure) spare tire

Fluids:
water (I think there's a total of 10l out there at the moment)
drinks in bobs (coffee, hot chocolate, koolaid, etc.)
washer fluid
transmission fluid
oil

Extra food:
trail mix
cliff bars
peanut m&ms
food in edc and bob (bars, MH, etc.)

The space between my ears:
Check and re-check the weather reports for your entire journey
file trip plans with responsible people
stick to the plan
top up all fluids before trips
keep gas tank above 1/2 full
change oil, brakes, etc. regularly
don't test the 4WD unneccessarily
dress ro survive not to arrive
drive defensively
snow tires
keep on eye on the rubber