I'm surprised that there hasn't been more discussion of the Costco COFFEE!
That alone pays for our Costco membership - 3 pounds of delicious, organic Rainforest blend for $15, whereas I'd be paying about $9 a pound for regular beans at the grocery store, and more like $12+/lb for organic fair trade stuff.
Ditto the previous poster on the coconut oil. Their jar is about 6# for $22. Regular grocery store is $10 for a pint jar.
We don't have kids, but we have dogs, and the dog food there is actually very good quality for the price. I've compared the ingredient list of the Kirkland brand dog food and it seems to be more actual food and less filler than some of the upper-mid range dog foods like Iams or Science Diet. Also, the dog beds are actually affordable, and big enough for our 50 and 80# dogs.
They also have good prices on gardening supplies in the spring. I've purchased all of our tulip and daffodil bulbs there, and they bring in really good quality 100' garden hoses for about 50% of the price of the home improvement store. Also, both gardening/work gloves and ski gloves are much cheaper there.
When the coupons circulate, I purchase my shampoo at Costco, so about $6 and I'm good for the year.
As mentioned, nuts are usually a good buy there, as well as olive oil. My husband goes through lots of blue cheese dressing and croutons, and they are less than half the price.
Tubs of salad greens and spinach are generally $1 less. Also, frozen berries are a good price per pound.