There are a ton of QR code scanners for iPhone so it almost seems silly to highlight one above the rest, but if you've gone through a bunch of them you know they're not all made equal. Scan is great because you launch it, it's immediately in scanning mode, and it can pick up a code within seconds. There is no BS — it just does its job.
Really, that's all there is to it. You launch it, scan the code (it even works from far away, out of focus, and at an angle), and it provides you with what's inside. If the contents are a URL, it just loads the URL in its built-in web browser. It does its one job really well, so if you need a QR code scanner this is the one to download. And as an added bonus, it's completely free.
Scan (Free) [iTunes App Store via OTW]