QR code reader for OS X 10.6?
An alternative to a native OS X application is an online QR code reader, such as 


ZXing 
miniQR (also allows you to use your iSight camera) 
Online Barcode Reader (also decodes barcodes; an added plus)
Patrick Wied QR Generator (also allows you to create QR codes).


All of these services are free. To use them, all you need to do is upload a screenshot, and the website will return the result.