Currently waiting in Apple's review queue and hopefully launching on the Mac App Store on January 6, QuickMark is a new app that will let your Mac "read" the increasingly popular QR codes. Thanks to the built-in iSight every Mac (which Apple is apparently rebranding as "FaceTime camera") will be able to recognize 2D barcode images or, if a Mac doesn't come with an iSight, load an image file and scan it. You can also create your own 2D codes, if you wish.
QuickMark's reader is capable of doing a bunch of stuff with a correctly scanned code, too. For example, the app can follow a web link with a Mac's default browser, send an email or add a contact to the Address Book. The app also sports a neat Skype integration as it can send SMS and make a phone call; if you don't like making phone calls and you're more for meeting your friends in person, the app can also navigate to a Google Maps' specific point of interest.
QuickMark for Mac should be available in the App Store soon.