RedLaser is very intriguing.
I picked it up and have been playing with it for a couple days now. It doesn't work on too many grocery or household items (the Canadian support is a bit wonky right now), but it works like a charm on video games, books, etc.
Next time I go to EB to "take a look around" I'm going to scan a few bar codes before I buy anything.
Edit: They also have a downloadable SDK if anyone's interested. Note that they have developer and licensing fees if you want to actually publish an app using their SDK.
Edit: You can add your own sites too!
www.redlaser.com/apps