I just downloaded RedLaser on my iPhone yesterday and is one of the coolest apps I've discovered yet. It uses the camera on your iPhone to "scan" barcodes from nearly anything and then will search the web looking for the best prices on the scanned item. The web search includes a link to Amazon which not only gives you pricing, but also reviews of the item.
I often use my smartphone (formerly my Treo) to look up reviews and prices of books I'm thinking about getting when I'm at a bookstore. This app makes this process completely effortless and turns it into a 5 second process. Brilliant!
Here is a search result from a 1 second scan of one of my microeconomic theory books:
You can purchase RedLaser for $1.99 on iTunes. For me, it was money well spent.
Below is a video showing how quickly it works. It my experience it works even quicker than this: