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A derivative of ZXing's Barcode Scanner version 4.2. The only modification is the addition of support for front-facing cameras, including support for devices like the Nexus 7 (which doesn't have a rear camera).
Note that front-facing cameras can be lower quality and less able to scan fine-featured barcodes, but coarse QR-codes scan just fine.
If your device has multiple cameras, a button will appear in the top middle of the screen, which allows you to cycle through all of your cameras.
If your device does not have a camera at all, this app will not work.