In addition to scanning 2D barcodes, Barcode Scanner can also generate a QR Code and display it on your screen. Then you can show it to a friend, and let them scan the barcode with their phone:

To use this feature, press the Menu button from the main scanning screen, and tap Share. Then choose whether you want to share a contact, a bookmark, an application, or the contents of the clipboard. A QR Code will be generated automatically. When you're done, press Back or Home.

To generate QR Codes from your computer, try the ZXing QR Code Generator.