Every single kitchen must have aluminum foil. I use this all the time- in the oven and out. I cover cooking casseroles or tent it over stuff. Or wrap up left overs- whatever. I find if I'm doing something in the kitchen and am out of foil, I'm at a loss. And thankfully I can buy rolls of it for a $1 at Dollar Tree!!!
I think every kitchen needs 4-5 mixing bowls in various sizes, and at least one should be glass so it can go in the microwave. I find when I bake I need one for dry ingredients and one for wet and then always one to whisk the eggs in before I add to the rest of the recipe. Mixing bowls are a must- and lots of them. I like to stack them, too, like the Russian Matryoshka dolls.
Every kitchen needs multiple amounts of 9x13 pans, preferably glass ones with lids. I like to make a pan of brownies and a casserole at the same time so I need at least 2 pans. I like to have a few extra just to be on the safe side. I like the Pyrex Portables the best. And they stack nicely, too.
Wooden spoons and rubber spatulas are necessary. I like LOTS of wooden spoons. I used them for everything. I like that they can be used with any sort of pan- glass, non stick, plastic. The wooden spoon doesn't scratch anything. (And it's good to smack your kids with, too, or so my mom believed!!)
Tupperware containers are a must for every kitchen. And it doesn't have to be the Tupperware name brand; I like the disposable Glad containers or even Rubbermaid or Ziploc. It doesn't matter but I like those to store leftovers or use them to carry my lunch. And they're also good for things like paperclips, rubberbands, and other desk/ office supplies. Or for sewing or scrapbook stuff.
Non-stick spray like Pam. No kidding. Love it. I've been cooking and been out of it and I've stopped in the middle of meal prep to go and buy some.
I'm a firm believer in bottle scrub brushes. I clean everything with these.
I like lots of towels because I believe in kitchen cooties so I like to use a different one to dry dishes and another one to dray hands and another to wipe counters and another... well, you catch my drift. I also have several sets of measuring spoons and measuring cups.

So, other than food, I think these are the essentials that make cooking easier and fun. I think I'm gonna go and make something.... I'm hungry!
Chef Maggie