I carry this first aid kit.
http://www.costco.com/Browse/Product.aspx?Prodid=11072475&whse=BC&topnav=&browse=
I really like it. It has a good selection of typical needs. I added a couple items to it to make it more rounded. CPR mouth to mouth damns, Snake bite kit and a Thermometer. Personal need items should also be considered if you have allergies to bee stings you need to add treatments for that.
My Club has a bonified EMT kit (several of our members are Fire fighters and LEO's)that goes on trail rides. Our club also has a defibrillator that goes in the trail leaders rig with the EMT kit. Took me dropping dead of a heart attack for that to get added. I wasn't with the club when it happened but several of us have known heart issues.
My club also requires a emergency information "green sheet". We require one for each person in the vehicle. It has pertinent medial insurance and medical condition information and contact numbers for next of kin so you know where to send the caucus.
We also have a mandatory safety and equipment list.
http://www.gbr4wd.com/download/GBREquipListjan03.pdf
As for the emergency roadside kits...I have as of yet to see one worth a damn. The Jumper cables are JUNK. The rest of the stuff in them isn't much better.
I would recommend you invest in good materials:
Long Jumper cables that are 4 Gage with good quality clamps.
Mag light or similar high quality flash light.
Cheap compressor but if you have a flat while driving chances are the tire has a bigger hole then a little $10 compressor will deal with. If you want a 12v compressor then buy a better quality one and skip those cheapies.
Small assortment of quality tools. Nothing is worse then a cheap set of tools that fail and you bust your knuckles when they do. Get a roll up pouch and a assortment of open end wrenches that fit the most common sized bolts on the truck. Jap trucks mostly use 10mm,12mm,14mm 17mm. Set of slip joint pliers and a pair of vice grips (you can pinch break lines shut if you cut one down. Pinch off heater hoses as well. Always usable as a second set of pliers.
I like the emergency hose repair kits. Its a couple sleeves and radiator clamps. Throw a foot of 3/4 radiator hose and you can cover most issues to get you to a parts store.
Handful of 7 inch zip ties, couple bungee cords and a couple ratchet straps. I have seen more then one truck drive out of the woods with a ratchet strap holding a transfer case in.
Couple road flares.
Spare flash light batteries.
A good 4 way lug wrench.....the wrench that comes with the truck is short and usually won't work with wide rims.
Jack that can actually get the truck off the ground. Most factory jacks won't lift a truck with over sized tires or a suspension lift. Usually a couple pieces of wood will make them work but find out what it will take before you are in need. If you factory jack lifts from the body and you have long travel suspension then you need a few feet of chain. If you chain the axle or suspension before you start jacking you will not need to lift the vehicle nearly as much. In the case of a highlft the less you need to lift the better. Just be careful not to pinch brake lines.
2.5lb ABC extinguisher with a GOOD bracket.
WD 40 and the junk is not good for lubrication. It is for drying out ignition components like distributor caps. WD stands for "water displacement". Great marketing makes you think its for anything else but it is helpful some times.
Duct tape....Million and one uses from holding a hood down to making a leg splint.