We use a marker almost every day in our kitchen. We label containers of leftovers, mark the date on freezer bags, and make quick notes to ourselves about recipes. It used to be that we never had a pen when we needed it, and then we realized – duh! – keep one in the kitchen!
We actually keep three different markers handy these days: a permanent marker, a dry-erase marker to use when we don't want it to be permanent, and a ball-point pen for making recipe notes.
We keep these pens in our silverware drawer so we always know where to find them and so they don't get lost in the chaos of other drawers. If your pens have a tendency to wander, label them with a piece of colored tape so everyone in your house knows these pens belong back in the kitchen.
Is a pen part of your essential "cooking equipment"?