I got married in June of 08 and I registered at 3 different places-- I just wanted to let you know that my best and most stress-free experience was at Bed, Bath and Beyond (the other two were Pottery Barn and Macy's. I also used Target when I registered for the baby, and that was a total nightmare.) BB&B gives you cash when you return things-- so helpful!
Some of the things I constantly use are:
Everyday dishes
Corningware
Pyrex bowl set
hand mixer
crock pot
toaster
knife block, every day silverware
coffee pot
cookie sheets
bedding
bathroom decor (soap pumps, trash cans, etc.)
Dyson vacuum cleaner
measuring spoons/ cups
Some things I don't ever use:
fondue pot
kitchenaid mixer (I know some people use theirs often but I don't, and I can't justify how much was spent on it that could have gone towards more things from the registry.)
bread machine
To answer your question though, I guess the ONE thing I couldn't live without from my registry is either my crock pot, my coffee pot or my Dyson.
Can I borrow your fondue pot-we are doing a fondue for Christmas Eve and I need one more
OP, I agree with the others, get a nice set of neutral China-either more casual or fancy depending on your tastes. I used my China a lot, but I got fairly inexpensive ($50/placesetting) so I would have enough. I still like my China but it has a light blue pattern and for non-holiday stuff it is fine. I inherited my Grandma's Currier and Ives red dishes and I use those more than my China now because they work for most holidays.
Also, when you register for dishes, flatware, glasses, etc. register for 12 or more of everything. 8 just isn't enough. We are a family of 5 and if we have one more family over, that puts us over 8.
One thing that doesn't get used all that often but is REALLY handy when we need it is our electric knife. It doesn't take up much room but it is handy for carving a turkey, ham, large roasts, etc.
We have a crock pot but I don't use it all that often. It is handy to have. I would register for one that has the insulated carrying bag because more often then not you will want something to use to transport your crock pot.
I have a lot of Pyrex dishes-the storage ones with lids. I use those every single day.
I also agree with getting a bunch of white towels. They will go with everything and you can bleach them to keep them looking nice. We got some from Land's End for our wedding almost 21 years ago and they STILL look good. Most fancy designer towels aren't worth anything-they fall apart too easily. Get some of the nicer towels from Target. They will last longer and work better.
Don't forget some of the basics like mops/brooms/swiffers. They tend to add up if you have to go buy new. When all of our friends were getting married, this was my go to shower gift, a cleaning bucket with a mop, broom, duster and other cleaning things.