These are the free QR Code readers for Android, Apple and Window phones. I use a QR Reader offered by Scan Me and I've used both (currently back on Android, I prefer the OS). I use QR Codes extensively throughout my marketing materials so it makes sense to offer the apps here for free.
Scan Me
Free QR Code Scanning Apps
Want the paid version (It's only £1.99 and offers more features) then visit the Scan.Me download page here here.