Kleenex (for you, not the car).
A few shop rags.
Flashlight.
Snow brush and ice scraper (winter).
Tire pressure gauge. Takes up virtually no room.
Small tape measure. Can't tell you how many shopping trips I've been on where having a pocket tape measure came in handy. How wide is that shelf? How tall is that planter? The only way to have it at all times is if it's in the car.
Speaking of which: Grocery store membership cards. I carry exactly two cards in my wallet, and I'm not carrying around another three or four semi-useless cards just for grocery trips, so I keep them in the car.
Custom windshield sun screen. Makes a big difference in the summer, but only if you're not lazy and you actually use it. I'm not a fan at all of tinted windows and their reduced visibility, but I suppose that in some harsh climates they may be a necessity.
I've carried glass wipes before, but they always dry out before I use more than a couple. Just clean your glass regularly and you'll have very little need for them. BTW, Invisible Glass is good stuff.