In addition to the above:
Duct tape (or gorilla tape)
Black trash bags
Ponchos
Emergency blankets / tarps (great for laying on wet ground to keep mud off yourself)
High quality flashlight
Orange reflective triangles
Lighter
Paracord
Gun & pepper spray are also good to keep in car.
zip ties and safety wire and vise grips are good to have. I have vise gripped a clutch cable, zip tied a throttle body open, and safety wired an exhaust pipe to get home (on separate occasions.)
You can't always stay with a broken down vehicle, so:
backpack
extra clothes- jeans, t-shirt, light jacket, old shoes (breaking down in dress clothes sucks), if you're married then 2 sets (imagine listening to your wife in heels complain for hours while walking...).
a couple of energy bars
Ask a doctor what you should put in a first aid kit, and he'll say medical supplies.
Ask Bayoushooter what you should put in a first aid kit, and you will get answers like:
"backpack
extra clothes- jeans, t-shirt, light jacket, old shoes"
"zip ties"
"Duct tape (or gorilla tape)
Black trash bags
Ponchos
Emergency blankets / tarps (great for laying on wet ground to keep mud off yourself)
High quality flashlight
Orange reflective triangles
Lighter
Paracord"
You want to know what to keep in a first aid kit? Ask a Registered Nurse or a Paramedic.
We have a few on the forum, but I don't think a single one has posted yet.
My $0.02, before you build a First Aid Kit... Buy one. It will have (almost) everything you could need in a situation that would require "first aid." Remember, "first aid" is not the same medical attention that a Paramedic/EMT could give, so don't go play hero.
Better Dave? :]
Just to clarify, I'm requesting input from all venues. Keep them coming.
I keep a crowbar (Stanley Wonderbar) as well. So far out of everything in my car, it is tied for most used with the flashlight haha. Well...maybe the SOG Multitool too.