A knife sharpener is used to hone the edge of a knife to keep it sharp. I know, you're probably saying "Duh, no kidding" And I know, a knife sharpener is hardly an "ingredient" but it's certainly something we need in our kitchen. This page is really not about all the types of knife sharpeners but more about what to do when you need one and don't have one. But while you're reading I may as well list a few of the main types of sharpeners available.
Sharpening Steel - The sharpening steel is a long, cylindrical tool with a handle. To use you drag the blade of the knife along the steel at about a 20 degree angle alternating sides of the knife blade. This sharpener stores easily in your kitchen gadget drawer. It's recommended that you sharpen your knife every few times you use it.
Sharpening Stone - The sharpening stone is a flat, rectangular piece of stone that you use in pretty much the same way you'd use a steel. You can read more about sharpening stone types and uses here. This is pretty easy to store too.
Chantry Knife Sharpener - This is my favorite although I'm sure most chef's might scoff at its use. It's easy to use, you just drag the blade through the two steels located in the middle of the sharpener. You slide the blade through 4-5 times and you're done. This sharpener was designed back in 1970 and it's pretty cool looking too. (This sharpener is shown at the top, left side of this page).
Electric Knife Sharpeners - These can be expensive and a little more tricky to use. The sharpening method is similar to that of the Chantry but it's easier to grind the knife blade incorrectly. I inherited one and it's pretty big and bulky and of course you need to have a ready source of electricity to use it so it's not much good for camping trips or power outages. Come to think of it I'm not so sure I would be using a sharp knife in a power outage.
If you don't have a knife sharpener and you're desperate, here is a trick I've used that works "fairly well". If you have a ceramic dish, plate, mug, anything, turn it over. You should see that the bottom rim of the plate has a rough edge. This won't work for just any dinnerware. Hold your knife at the same 20 degree angle you'd use for a steel and slide the blade from the tip to the handle then alternate sides. You'll need to do this at least 8 -10 times per knife side. This won't sharpen your knife perfectly but it should help quite a bit. I was staying in a condo recently and the only knife available was dull as a butter knife. I tried this handy tip and it worked pretty well.
Alternately, you can use an emery board. Hold the emery board at a 20 degree angle and move it from the tip to the handle, then turn the knife over and repeat. I've yet to try this one but I'll let you know when I do.