Multi-Tool
Small pocket knife
tire gage
LED Flashlights (the AA flashlights have Lithium batteries, the large D-cell flashlight has standard Duracells that periodically get rotated out)
Some extra cash hidden in my car
Basic tool set
cheap gloves to keep my hands clean
1 quart of oil (MaxLife Full Synthetic 5w30)
1 oil filter (Fram Ultra)
Long pair of quality jumper cables (and I do mean quality, I've actually been complemented on my jumper cables before)
Fleece sleeping bag that can unzip into a blanket
Roll of Duct Tape
Roll of rope so I can tie down something on the roof rack such as a Christmas tree
I keep an old fleece pull over or zip up hoodie tossed in the trunk along with an old hat incase of cold or rain
This one is dumb, but I keep my old wipers in my trunk as well. I've been caught in snow/rain and discovered that my wipers were terrible, or had the edge tear off one. So when I replace my wiper blades, I toss the set I remove in the trunk (and trash the previous set that was in the trunk). That way if I get caught with bad/torn wipers again, I can toss on the old set to at least get me home or to a shop where I can get some new wiper blades.
Similarly, I had to replace my headlight bulbs when one burnt out. I replaced both bulbs and threw away the bad one but keep the one that was still working and put it in the trunk. If I find myself in some completely unlikely situation where both bulbs go out, I can at least put the old good one back in to get me home or to a shop to buy more.
I'm sure there's more, but without going through my car I can't remember everything.
It seems like a lot when I write it out, but everything is neatly packed away out of sight. Some of the small things that I use often enough (tire gage, flashlight, multi-tool, etc) are up front in the glove box or center arm rest console.
Everything else except for the fleece sleeping bag and pullover/hoodie fit in the under floor compartment of the rear cargo area. On either side of the rear cargo area are a small compartment about the size of a gallon of milk. I keep the fleece sleeping bag rolled up in one of those compartments, and the pullover/hoodie is literally just thrown in the cargo area. So the pullover or hoodie is the only thing you'd see if you looked into my car. You'd see the fleece sleeping bag if you looked for it, but b/c of the space and how it sits flush it's easy to overlook.
