Other than the stuff that's always in there... spare tire, tool kit, medical kit, we usually only carry luggage. When the kids were small we sometimes packed a cooler with drinks and sandwich stuff.
In the Winter I always try to put a D-handle plastic-blade scoop shovel back there. Most of the rest is always in there: jumper cables, super-capacitor pack jump starter, 12 volt tire inflator, 3 orange-flasher magnetic emergency lights (double as flash lights), a couple of microfiber rags, a pair of old "mud shoes." Spare, jack and tools came with the car. A few of the old plastic grocery bags. Remainder of a roll of duct tape.
I always have road flares, spare, hand cleaner, rags, etc. I also carry paper maps. Back in the day, I used to carry an extra fan belt, points, condenser, distributor cap and tool kit. These days I don't mess with that stuff...there's nothing mechanical I could possibly fix, either at roadside or at home. I carry my AAA card instead.
I'd like to pretend that I'm more responsible than I am, but sometimes I don't even check to see that I have a spare tire. Since I rarely get flats anymore, I usually do have a spare tire. Although I'm perfectly capable and equipped to change a tire, I can handle that with a cell phone and my AAA card. In the winter, even on shorter trips, I try to remember to place a wool blanket in the car; I have one in plastic for just that purpose. It's a thick, 100% wool blanket, so I think I could survive almost any Maine winter night in a car. Although I will have either my Garmin or my iPhone for GPS, I carry a Maine Atlas for trips within the state when traveling alone because I like to turn off onto roads I haven't been on before. When my wife's along, she doesn't let me get off the Interstate unless it's absolutely necessary. I should carry a small shovel, which I have, and a flashlight, but I usually forget to take it. One might think I could just leave them in the car, but my wife often unloads the trunk; I'm not sure why; maybe she puts bodies in the trunk when she's off by herself, I don't know.
A couple of beach towels, a folding chair, old sneakers, a tarp, a gallon of water and a roll of paper towels, plus the usual jumper cable, spare tire, jack.
Oh yeah. I normally have at least one of those thin rolled up fleece blankets that sell very cheaply in the back of the car. I'm starting to think a small plastic tarp makes sense as well.
We carry a lot of stuff in the car in the winter here, as breaking down in winter here could mean death at -40 F./C. In the summer we carry much less. There are flares and the usual spare tire in the car. In the truck I carry about the same year-round, but we don't take it on long drive. I do have basic tools and fluids, and an emergency sleeping bag plus tarps and ratchet straps and rope.