ZXing - Barcode Scanner for Android
QuickMark - QuickMark makes everything possible.
Obsqr - obsqr is an open-source, fast and lightweight QR scanner for Android.
QR Droid - QR Droid Zapper
NeoReader - The NeoReader is a universal barcode scanning app that transforms your mobile phone into a barcode scanner, allowing access to mobile web content by scanning codes from virtually any medium
Scan QR Code Reader - Scan is the most straightforward QR Code scanner available for iPhone and Android.