OK. No votes. I get to stick with my first pick at a sample Web Service. Let's move to step #2: "Simulate":

1. Postulate - Take a guess what the market wants.
2. Simulate - Don't write the full functionality yet. Write a skeleton that emulates the product.
3. Demostrate - Show some potential customers and some non-potential customers and get their feedback.
4. Integrate (feedback) - Filter what comments are good and change your simulation.
5. Iterate - Go back to step 1 or step 3 and do it all over again.

My choice of example Web Service:

Locator service of family members - Where's dad? Where's mom? Where are the kids? (Maybe GPS coordinates or a map with icons)

Here's some skeleton code:

// Locations are stored as simple (x,y) coordinates for a map for each family member

public class FamilyLocation {
  String name;
  int x, y;

  // Web Service Methods
  String getName();
  Dimension getPosition();
}

The above is pretty straight-forward and is all we need to simulate the Web Service for now. The idea is to get a prototype interface done quickly and let it evolve instead of working on it for years to try to get it exactly right the first time. Software is a living, growing thing--especially Java software. ;-)