Google Goggles (part of the Google Search app) snaps a picture. If you wait for it to focus, it does it on it's own. Or you can force it by tapping the camera icon. That's the only app that I know of that takes a picture then scans. Of course it does far more than just bar codes. It will also try to match with online images, with varying degrees of success.
I'm not fond of it's product lookup results. They are just the usual Google choices, but if you like Google Shopper you can narrow down to those results:
However the Google Shopper app will get you there faster. It doesn't take stills, but seems to scan faster than Goggles, at least in my experience. And the UI is pretty nice.
I also have ShopSavvy. It doesn't take stills either. I do like the interface, and usually the results. They are getting a bit pushy about getting an account, but it's ignorable if you don't' want the extra list tracking features.