Data is Free

For my money one of the best apps on the market is Nike+. It simply has it all:

1. It does something useful - it tracks how far you ran and how fast you ran it.
2. It launched with a built in user base of millions of iPod users.
3. It drives sales - when you finish your run and plug it into your computer it takes you to Nike.com where you can buy more shoes.
4. It is fun to use, and aesthetically beautiful.
5. Because it is fun to use, and because it sets goals, it encourages you to run more. And if you keep running it probably means you are going to need a new pair of shoes.
   
6. It is generative: users can set their own goals and blog on the site.
7. Finally, Nike must be getting crazy amounts of data from the app. Given the data Nike is gathering, the valuation of the app has to be the value of the sales generated by the service plus the value of the data on the users. Though Nike is limited in how they can monetize that data beyond the site, I'd still ballpark that value in the neighborhood of $5 to $10 per user.