I carry tools, spare driveshaft U-joint, spare axle U-joint, serpentine belt, water hoses, heater hoses, air filter (probably not necessary), fuses, lug nuts, container of spare nuts and bolts, lots of tie wraps, duct tape, electrical wire and a couple of connectors, JB Weld and that putty type epoxy, a bunch of different size hose clamps (good for emergency repairs of many types), bailing wire, tire repair kit, Fix-a-flat, starting fluid, oil, gear oil, Bars Leak, ATF, brake fluid, power steering fluid, water, siphon tube, funnel and probably a few other things lying at the bottom of the tub. Everything except the tools pretty much fits in a single RubberMaid tub and the tools all fit in a small toolbox. I've also started carrying one of the "emergency start" batteries. My old Jeep had dual batteries and I didn't worry about it but I only have one battery in the JK and I see a dead battery as being a single point of failure that isn't that unlikely - $50 is cheap insurance.
One thing I've always tried to do is use the tools that I carry in the Jeep whenever I'm working on the Jeep at home. That way I find out if I'm missing something that I might need on the trail.
Crap, no wonder my Jeep is so heavy.