All the practical things have already been taken really, (ie video games, pc's, music etc) so you'll have to think outside the box...besides, it's always the websites that score at least a 150 on the batsh*t crazy meter that are popular, you gotta think outside the box...
With that in mind, I think you should make a site that is completely devoted to something like
- The letter Q
- Light Switches
- Rosie O Donnels Pants
- Winston Churchills Cigars
- Dental Floss
- Bannana Peels
Or a off the wall review site...like review the pros and cons of stuff like
- Glass cups as compared to plastic ones, which is better? which keeps your drink colder? which has a longer life? do hand washing cups as compared to dishwashing cups give them a longer life?
- Toasters versus Toaster Ovens? which cooks your food better? which is more convienent? can you really get radiation sickness from them? which is better for cooking grilled cheeses?
- Peanuts to Pecans? which do YOU like better?
- World dictator beauty pageant, where users vote in which historical despot is the greater stud. Will it be Hitler? Chavez? Ceaser? Castro? Pol Pot? or how about that chap Stalin?
- Who has the worlds sexiest mustache? with different categories for men as well as women!
- A Website tracking the development, struggles and triumph of the Baked Bean
- Why are the buttons on men and womens shirts on opposite sides? (something that I've always wanted to know)
I haven't checked if any of these exist, all these are just off the top of my head but they are some food for thought anyway