What's the most useful thing on Thingiverse? IMHO, it has to be Makealot's Spiral Cup. I've probably made a hundred of these things. My daughter gave them away to practically every kid in her class and I have them all over my house and at my office at work. It's the perfect size to stick stuff in and it's a beautiful object as well. A 3d print does not have to be complicated to be useful. If I have idle time for my Makerbots, I can always print Spiral Cups. They won't go to waste.