How to read barcodes with the camera on Android?

2016 update

With the latest release of Google Play Services, v7.8, you have access to the new Mobile Vision API. That's probably the most convenient way to implement barcode scanning now, and it also works offline.

From the Android Barcode API:

The Barcode API detects barcodes in real-time, on device, in any orientation. It can also detect multiple barcodes at once.

It reads the following barcode formats:

• 1D barcodes: EAN-13, EAN-8, UPC-A, UPC-E, Code-39, Code-93, Code-128, ITF, Codabar
• 2D barcodes: QR Code, Data Matrix, PDF-417, AZTEC

It automatically parses QR Codes, Data Matrix, PDF-417, and Aztec values, for the following supported formats:

• URL
• Contact information (VCARD, etc.)
• Calendar event
• Email
• Phone
• SMS
• ISBN
• WiFi
• Geo-location (latitude and longitude)
• AAMVA driver license/ID

It's not built into the SDK, but you can use the Zxing library. It's free, open source, and Apache-licensed.

The 2016 recommendation is to use the Barcode API, which also works offline.