RedLaser, a new app available via Cydia, is very cool. It's a barcode reader for the iPhone - and it works even on the 2G and 3G iPhones, without auto-focus. Reading straight from the camera feed (no need to take a photo, just hold it up to the bar code and wait a second), all 3 models of iPhone can run RedLaser. Oh, and its free... which is a nice price.
The app lets you do online price comparisons using Google's db, and view items in Amazon.com. There's all sorts of extra options available as well - and way more possible with their SDK. Imagine reading reviews on scanned books, beaming movies to your TiVo, adding milk to your grocery list... possibilities are endles.