One thing I hate is every store seems to have a discount card that is fighting for space in your wallet or on your keyring, Card Star helps with this. I have used this app for at least a year and love it. Card Star allows you to input your discount card number into the app and it creates a barcode that can be scanned. Kroger for example is a place I shop all the time, so I put my Kroger card into the app and now at check out I have them scan the phone like I would my discount card.
The app has many stores already loaded into it but even if the store isn't in the app you can list it as other and put your info into it. A couple of things I've noticed using this app, first is if they don't have a hand scanner it will not work. Food Lion for example only has the flat scanners and I never can get it to work there. Also it works much better if you rotate it to landscape mode.
That's all I've ever used this app for and believe me that alone is worth it however the app does more. When you click on a store, let's say CVS, on the bottom is an info button that will give the number to customer service, CVS's home page, and a store locator. Also under the extra's button you can check into Foursquare and then there's a deal button that, well you guessed shows you deals that they store may have. I don't use these features so I'm not sure how well they work.
This is an app that I would say is a must have for all iPhone and iPod owners. No longer do I have all of those plastic tags hanging off of my keychain (I never liked them but the wife would keep putting them back on my keychain). The app is free.
Free in iTunes