For a family of two, the mini-french breads in the bakery (sold in packs of six) are great. They are the perfect size, great quality, and easy to freeze individually. Fun post!

Gasoline! This week I paid $2.25/gal. I paid with my Costco Business AmEx card that gives a hefty percentage back on gasoline purchases (yes I pay it off in full ALWAYS). The cheapest I could find fuel at other stations was $2.29/gal cash, 2.39/gal credit.

Free range eggs are SO cheap at Costco! I was in a hurry yesterday and paid $4! At Costco they're somewhere around $2.

Also, they carry some tasty organic bread (my boyfriend thinks its too chewy, I like it). He goes there for electronics, I always stock up on eggs!!

Bread flour, chocolate chips, YEAST (by far the best deal per ounce at Costco), spices, milk, cheese.

The Costco rotisserie chickens are much cheaper, and much larger, than at any of the grocery stores in my area - and much yummier. I can make multiple uses of the leftovers, such as really quick chicken fajitas, for even bigger savings. Flour tortillas, white or whole wheat, are MUCH cheaper than any grocery store prices in my area, and their large quantity stores for a long time in the fridge with no ill effects.

I make my own pizza (and breads occasionally), and I've found Costco is the cheapest place to buy yeast, sliced pepperoni, as well as shredded cheeses - parmesan and mozzarella, as well as cheddar. The cheeses come in 3 - 5 lb bags, which I divide into smaller quantities and freeze the extra. There have been comments that frozen then thawed cheese loses some quality, but I haven't found this to be true, especially if you're using the cheese for cooking.

Costco is also the only place I buy gas. We have 3 different gasoline brands in our small city, including Costco, and their price is always cheapest, even when the prices spike upward. A survey of surrounding cities has proven Costco gas prices are regularly the cheapest in the whole area.

great mentions!! I love the soymilk!

Mercedes

A video game system, unlike computers and some other electronics, is a returnable item at Costco. (So are the video games). Xbox is notorious for failing. Mine has twice. Instead of sending it to Microsoft support, I boxed it up & returned it to Costco, then went and bought a new one at a greatly reduced price since the price is always dropping. I've done this twice.

Okay folks I haven't seen someone mention the tortilla's. A STEAL at Costco! I am feeding 7 people and you can't beat their price for those!

Other big savings items:

Dog and cat food
Sourdough bread rounds - 6 for like $3.50. I put the garlic spread (big jar at Costco for $4 that you blend with butter and the jar lasts FOREVER) and use that as my garlic bread on spagetti night.
Butter
Eggs
Coffee! WAY CHEAP! Its worth a trip just to buy that!
Drugs - pain meds, allergy meds, etc. Can't beat their prices especially if you buy the Kirkland brand.
Diaper and Wipes (though we are past having to buy those!)

I love Costco! Saves me family a ton of money!

Kalamata Olives
and
Bully Sticks (though these aren't alway there)

I have not priced the organic milk, but the Kirkland brand milk has no rBST (artificial growth hormone) and it is sold much cheaper than the local grocery stores (which all sell milk with rBST).

I buy the following items at Costco for less than at my grocery store:
cheese (block and shredded)
butter
yeast
chocolate chips
honey in bulk
eggs
Abuelita tortilla chips
ground beef (quite a deal!)
frozen strawberries
organic baby carrots (5 lb bag - too big for many families but not for mine)
bananas

I hate shopping at Costco because I really don't like the fact that they look at my receipt and cart every time I leave the store as that just slows things down. There can be quite a line at the door to get out.

But the deals are quite worth it.

I second the rotisserie chicken idea. 3 lbs minimum vs. 2 lbs or less elsewhere. Fantastic deal for a quick and easy dinner.

We shop at Costco nearly exclusively, but one of my faves is their men's jeans. $13 for a VERY simple pair of jeans (5-pocket), no fancy designer add-ons. This is the kind of jean I actually prefer, and it's hard to find for anywhere near that price.

Here's another blog with a good listing of what people like to buy, either for savings, value, flavor, etc., The really interesting thing is looking through the comments to see what you might be missing out on. I found a number of things that I hadn't even thought of looking for at Costco.

The blog author has also done a price study comparing Costco to three of her local supermarkets in regards to basic food products like bread, milk, butter, eggs, etc.

For every person, the Costco experience is different.
You mention the seafood weekends. The $8.99/lb shrimp are a size that Whole Foods sells for $15.99 on a good day.
The cheese stix (motzarella) are $2.66/lb, less than half the supermarket price.
The secret to my Costco success is knowing prices, and tracking the unit price of the items I buy. TP and Kleenex at Costco? Better than supermarket, but not as good as BOGO sale with coupons. Same with some detergents, dish soap, etc.
When my daughter was a baby, the diapers and wipes saved enough to pay the membership in the first couple months.

Costco keeps looking for the best deals, so many items won't have a huge selection, you may also fall in love with an item only to find they no longer stock it. So for those, I just enjoy it while they have it.

The Kirkland garbage bags are awesome. Also, the Kirkland 600 thread count sheets are better quality than the super expensive ones from Macy's, Bloomies, etc. They are not cheap ($69 queen) but they are AMAZINGLY durable and soft.

I suspect that prices may vary from store to store. I've notices when traveling that Costco will have items particular to a certain region, for example, the Texas Costco had Spanish/Mexican foods on the shelves that I never see up NorthEast.