Ammo (mainly 7.62X51 and .223).
Soap.
Isopropyl alcohol.
Thought of another thing actually do stockpile: Air Monarchs. The most comfortable shoe I’ve worn, you can even hike in them (or even jog if need be). I am afraid they will change the style, so every time I’m in a shoe or sports store I stock up on as many Size 13’s as they have. Unfortunately, I never get a good stockpile going because they sometimes don’t have any size 13’s, and I am only in a sports or shoe store a few times a year. Right now I only have 2 pair (other than the ones I already wear), which will only last me for a year at my current rate.
For me, Land O Lakes Lightly Salted! When it is on sale for $2.99 a pound I fill the freezer for future baking opportunities.
Oh, and I forgot I do build a Kingdom of Soda in the garage during the summer. When 2-liter bottles are 99 cents, I make multiple trips in and out of the store buying the limit each time.
Booze is why God invented online shopping. It’s brought some interesting things into the house.
t.p.
cigarillos (I may take up the pipe.)
Russian-like vodka.
off-brand colas
Smoothies! Those damnable, hard-to-find V-8 Tropicana Pina Colada ones and the even harder to find squeezable cream of coconut. I’d prefer green olive juice but I can’t eat that many olives. Vodka and green olive juice is heaven on earth.
BOLD Pilot G-2 refills.
notebooks, legal pads, Post-It Notes, index cards…
onion dip
It seems that many of us have the gene that makes us buy notebooks, legal pads, pens, mechanical pencils, etc. I’ve always loved school/office supplies.
Stockpile:
Toilet paper - I’ve always kept an unopened 4 pack or two. Since I’ve moved to an area with no 24 hour grocery stores, it’s gotten out of hand. (pause…) I went and counted for you. I have 82 unused rolls.
Notebooks, legal pads, pens - yes, please, I would like some more.
Tape - Scotch tape, electrical tape, packing tape, duct tape. I like tape.
Cat food - Canned and dry.
Have begun couponing, so now there is:
Shampoo and conditioner.
Body wash and bar soap.
Deodorant.
Collect:
Pretty rocks. Amethyst, agate, azurite, dolomite, sodalite, rose quartz, shiny bits of mica. Chunks of marble. One beautiful big chunk of red and black granite. Lots of rocks. Rocks rock. 
I have two big rocks that I don’t even know what they are. They’re my “Colorado rocks,” because that’s where I found them. :o
Certain candy, like Reese’s cups and Butterfingers, which are not available here. We usually stock up during our home leave and occasionally get friends/colleagues to bring additional supplies.
Ammo - 5.56 and 9 mm.
I keep 2 shopping lists on the go at all times.
- stuff I’m actually out of and need
- Stuff that I am not at my limit of and can buy when the sale is good enough.
List 2 generally consists of Paper products (paper towels, napkins, tissues and toilet paper) and food with long expiry dates. I don’t see any reason to pay full price for something that goes on sale regularly when a little planning will save me money.
In addition to most of the mundane stuff already posted, I always make sure I’ve got swamp cooler parts on hand. The pumps always seem to burn out at 6pm July 3rd, or an equally inconvenient time.
I’ve started grabbing all the .22LR I can get my hands on (at reasonable prices).
Me, too. My high school age niece and nephew are always pleased when I have just the right paper, file folder, or whatever they need for a school a project. They refer to me as the God of Stationery (a very minor deity, to be sure, but still…) or, more literary fashion, Lord of the Files. When they were about 8, they thought my manual typewriter was so great because “it printed without a printer and at the second you hit the key.” And now typewriters are cool again. I’m saving my wide ties, too.
Cat food. If I ever run out, they’ll eat me.
Nice. 
Mobil 1 5W-30 full synthetic motor oil when its on sale, sometimes $20 or $25 off. It costs twice as much in Canada as in the States.
But not too much, since the oil changes are extended, and if I crack up this car or everything but the engine falls apart, the next car likely would take a different viscosity.
I stockpiled a pile of incandescent light bulbs, the 60s and 100s for over the back and front doors — the curlicues are laughable in winter — and the 60 and 40 watters for a small table lamp tri-light, a tall floor lamp with a dimmer that takes two and the bedside lamp.
Forgot about the hotel soap. Definitely stock up on that. Strangely, hotel shampoos and my scalp are incompatible, but only once have I had an untoward reaction to hotel soap, now that Dial (blecch!) is no longer common.
I stockpile everything when it’s on sale, because I manage a household of 8 people and 3 dogs. We go through prodigious amounts of stuff - from toiletries to toilet tissue. I get twitchy if there aren’t at least two or three “spares” of everything.
The thing I keep handy that I haven’t seen anyone else mention? Plumbing repair supplies. Several feet of PVC pipe, a couple of the attachment sleeves, a fiberglass quick repair kit, and the correct glue/cleaner stuff to hold things together. There’s nothing like a busted water pipe at 10 pm to teach a homeowner the wisdom of having some plumbing supplies on hand!
I also have a big box of plastic shopping bags, and plastic containers that originally held sour cream or such, because those are some handy “trash” - send soup home with my dad, no worries about getting the container back, use grocery bags to line the bathroom trash bags or pick up dog poop or tie up other stinky trash.
And baby wipes, even though all of the children are finally out of diapers. I keep wipes in all of the vehicles - handy for getting grease off my hands if I’ve had to monkey around under the hood, or for wiping messy faces and hands on small people, or blowing noses, or a quick polish if my shoes look disgraceful. There’s a pack in each bathroom, because life can get messy. There’s an emergency pack in the drawer of the dining room sideboard - in-laws on the way over? You can do an amazing amount of furniture cleaning/polishing with a couple of baby wipes! And I use them instead of tissues when I have a cold or it’s a month into allergy season - my nose is happier that way. The cheap unscented ones from the dollar shelf at the grocery or from Big Lots work just fine, and the cost comes out about equal to buying Kleenex or paper towels for similar use.
Ammo: .22LR when I can find it at a decent price/brand and .222 because it’s so hard to find and I have a Mini-14 that’s chambered for .222
Deodorant
Mirado Black Warrior pencils