I don't know about my favorite household item, but the one that gets used the most is my little salad spinner. I used to use the colander more, but the spinner gets the water out of herbs and lettuce much better than draining them in the colander. Plus, the bowl is clear plastic and I serve the salad in it. Heck, sometimes we eat the salad out of the bowl and skip the salad plates/bowls.
My cheap blender is the second most used item. I make smoothies and creamy salad dressing in it. DH keeps trying to get me to buy "a good one." I'm waiting for this one to die first, but it keeps on working. Seriously, I think I paid $15 for it 5 years ago and I use it a couple times a week.
A Swiss friend brought me a peeler that works better than any other I have found in this country. I love it, but I don't use it much. DH likes the skin on his potatoes and carrots.
Two items I bought (long ago when I had a bigger kitchen) that I seldom use, but just can't part with are my crock pot and food processor. It takes as long to clean the food processor as it takes to process by hand, so I seldom excavate to the back of the cold storage cabinet to use the food processor. It's my own fault for putting it so far back that it takes 5 minutes to remove everything in front of it in order to get it out, but I only use it once or twice a year and all the stuff in front gets used much more often. The crock pot seldom gets used because DH is phobic about leaving anything on when no one is home.