iTunes credit is one of the gift certificates you can choose. On the printed receipt that the Coinstar machine gives you is a code that you can enter in the Redeem page of the iTunes store. Since that loose change was just sitting around not doing anything in your house, it almost seems like you are getting free credits.
Don't forget that iTunes credit is also good in the iBookstore and the Mac App Store.
If iTunes credit isn't your thing, the Coinstar machines also gives gift certificates for a variety of other stores such as Amazon.com and Starbucks. You can see he full list here.