For tools, I have pretty much what JimK-NY has. I also have a jumper box (actually 2 now - one I use for charging phones, kindle, laptop, etc.)
Other than that:
a basic first aid kit. Add in a couple ACE bandages, tick tweezers, bottle of rubbing alcohol and peroxide.
A couple lanterns and flashlights, water pitcher or bucket - if your batteries die, you can keep on camping with these items on hand; just use the campground water (if you're boondocking, this won't necessarily work - in that case, having a water filtration system might be a good addition) by way of the water pitcher to cook, flush, wash, etc.
Small barbeque or campstove or fire grill - if your propane system dies, you can keep on camping by cooking outdoors.
Soft-sided cooler - if your fridge dies, you can just go get ice and keep on camping.
A credit card and some cash on hand is good to have also. If you need help, you might not get it from someone who can take a credit card - so having some cash is good. I once ended up paying an independent tow truck driver by using my credit card to put the money directly on his kids' school tuition bill:
Bringing your imagination is number one. It's amazing how you can make things work in a fix with a little of that.