iOS: Scanning QR codes with your iPhone should be a simple affair, but even our favorite app for the job has added a signup process and other features that aren't really necessary. QR Code Simple offers only what you need to scan codes without pointless additions.
If all you want to do is scan a QR code, you can stay on the central scanning tab of the app and just point your iDevice's camera at the code. It will pick it up really fast if you're in a well-lit area -- so fast that I had to try five times just to get a screenshot of the interface.
If there isn't adequate light, the app has a flashlight button to enhance scanning. It also allows you to access your history, visit any URLs in the code, and email the contents of the code to a friend or just copy it. There's nothing much else to it, but that's arguably all you need in a QR code reader.
QR Code Simple (Free) [iTunes App Store]