I think I have a frequent flier account for just about every U.S. airline that's been in the air over the past 10 or 15 years. It's become habit. If I fly any airline for the first time, I'll sign up for their member's program whether or not I ever intend to fly that airline again. The end result is that I've accumulated quite a few accounts with just a few thousand miles.
Webflyer.com has a great idea for putting those unused miles to work - donate them to one of the relief organizations working to get boots on the ground in areas affected by the Indian Ocean tsunami.
Here's a link for more information.