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I sometimes give baskets of all those little gizmos that have limited uses--melon ballers, zesters, those garlic peeler tube thingies, mise en place bowls, etc. and a nice cookbook (How to Cook Everything is a favorite).

How about a nice non stick fry pan

Not food related, but I handed out a bunch of disposable cameras to people and collected them after the reception, had them developed, and put them in an album for my sweet little couple that got married and would never use anything they had registered for. 6 years later they still bring out that album, everyone loves it.

I'm 21 and a total food geek. I'll second the good knives and add a nice BIG cutting board, baking pans/trays, mixing bowls, small utensils - whisk, spatulas, tongs, wooden spoons, etc.

a case of good extra virgin olive oil -- get them hooked and used to the good stuff.....

oh man.. missed the under $100... doh!
how about cook's illustrated runner up enameled dutch oven, by Tramontina? or the one by Lodge?

otherwise, i'm going to go with the mixing bowl people... a nice stainless steel set, with lids.

Another vote for a Le Creuset dutch oven. It's the first thing I thought of.

The knife idea is also great. If you go that route, buy them a sharpener device and a steel as well. Otherwise, it will soon be just another dull knife.

if money were no object, i'd say an all clad three quart saucepan. hardly a day goes by that i don't use mine.

in your price range... hm, let's see... i also use my convection/toaster oven quite a lot, especially in the summer, for things like baking potatoes, toasting pine nuts, and roasting beets. i also absolutely love a viciously expensive large villeroy and boch mixing bowl i splurged on one day at macy's that has a pouring spout. i haven't spilled a drop of strained soup stock since i bought it.

i also like the idea of a really good knife. or how about a few small high quality things, like a lodge cast iron skillet, a microplane grater, a set of nested glass or ceramic bowls, a good pepper mill, and a gift certificate to penzey's?

Also, a pan or cookware that can go straight from the stove top into the oven, will be indispensable.

when i was a young foodie, i got a food processor as a gift and i still use the same one today.

but the kitchen item that has most changed my life in the last couple of years, is my giant awesome chefs knife. Once you use a good knife theres no turning back. id rather cook a meal with a good knife and flimsy pots and pans, than good pans and a cheap knife. maybe give it with a book or video of proper use and care.

@vegemite: I've heard about the coin negating bad knife luck, too! My parents always give a knife block (well--when it's a couple we as a family are very close to, not EVERYone gets the knife block) with a coin taped to each knife.

A guest at our wedding got us two Global knives: a santoku and a bread knife. We had registered for a wusthof set, but I'm totally sold on Global now. We use the santoku every day.

They're pricey, but one or two is enough. It was a perfect gift.

Just got married three weeks ago! We love our knives and bamboo cutting boards and KitchenAid mixer and All-Clad pans and fish spatula. I registered using Cook's Illustrated as a guide.

I didn't hear about that superstition until AFTER the wedding. I'm not too worried about it =)

We also received a few cookbooks we didn't register for: Le Cordon Bleu one from W-S, a dessert book from the same and "I Like You" by Amy Sedaris. The last one is hilarious, the first is incredibly cool.

For my 24th B-Day my parents got me a nice set of knives (at my request). My mom forgot to attach a coin, and I immediately sliced my finger cutting a bagel. She gave me a penny, I gave it back, and I think I have only drawn blood one other time in 8 years. You give the coin so they can give the coin back to you and then they are "buying" the gift (bad luck to give something that can injur is what I always heard).

The gift boxes from Penzey's spices are great - they have a good variety (starter, baking, grilling, even wedding), and they come packaged wtih whole nutmeg and bay leaves, with a card that describes why - I only remember that you must keep a whole nutmeg in the house at all times, but I don't remember what happens if you don't!

definitely a le creuset. maybe a waffle maker and a french press. or a pizza stone with good olive oil and a stand mixer.

I like the idea of random small items. I only started to get serious about cooking last October when I moved to my current place. I had basic pots and pans, dishes, a crock pot and a rice cooker. Here are things I didn't know I would need and I've purchased or put on my ever-growing wish list to pick up as I have the money: Microplane, fish spatula, a spider, small prep bowls, fine mesh sieve, pepper grinder, silicone pastry brush, good knives, and various spices that I didn't use when I relied on take out and packaged foods. (Or before that when I usually lived with someone who did all the cooking.)

Larger items that could be found for under $100: skillet that goes from stovetop to oven, grill pan and toaster oven.

My favorite Christmas gift last year was a dutch oven - given to me by my sister who loves to cook and knew how indispensible it would be as I learned more. It never leaves my stove top.

i'm 24 and i will also second and third the knives. once i tried my co-workers wusthof knives, i was amazed.
also a good cutting board, as well as something that goes stovetop to oven (dutch oven or a skillet that can do that).
also a 10 or 12 inch cast iron skillet. i looove mine and do everything in it. so much that i bought a two-range reversible lodge griddle, which is perfect for anything from breakfast (pancakes, french toast, hash browns) to dinner (searing steak, fish, burgers...)

$100 doesn't buy much LeCreuset or All-Clad, but, as luck would have it, retailers are currently offering the All-Clad 12" stainless fry pan with matching lid on special right now for only $89.95, a great deal.
Knives are matter of personal preference, I think (I like Henckels Four Star), but someone that has nothing would certainly welcome one good 6" or 8" chef's knife.
Failing that, how about a couple of pieces of nice, Emile Henry ceramic bakeware, or an Oster blender, or a John Boos cutting board. I also love my Jaccard meat tenderizer and my Unicorn Magnum pepper mill (under $50 each).

Cast iron skillet! Not only is it the greatest cooking utensil ever, but it will last them for their whole marriage, even if they're married 60 years!

As everyone else has said, really good knives. I'd get them some Kyocera ceramic paring knives - they're AWESOME.

Regarding the knives: I'm getting married in a few weeks and there are LOTS of knives on our registry, except no one has purchased them yet. Maybe they've heard this bad luck thing?

As for what to give them, I vote for an excellent pan (such as an All-Clad non-stick skillet) OR a good knife + cutting board + food products - maybe some excellent imported dried pasta, a can of San Marzano tomatoes, a bottle of good extra virgin olive oil, and a recipe for a simple - yet better than anything they've ever eaten - sauce?