One of my favorite and most used apps for iPhone is RedLaser. The main reason is that it has saved me a ton of money since I first downloaded it for 99 cents.
If you're not familiar with RedLaser, you simply launch the app and then hold your iPhone so that the bar code of the item you want to scan is in view of the iPhone's camera. Seconds later it does a lookup to the internet and shows you the prices that the item is selling for at various stores and internet sites. My favorite moment was when I used it to scan a cordless phone package at Best Buy only to find that same cordless phone on sale on BestBuy.com. Moments later I showed the scan to the cashier and she price matched.
At any rate, if you're still not sold on RedLaser here's another reason: it's now free! According to eBay, the company has acquired RedLaser and as a first step is making it free. Next eBay will integrate RedLaser's technology into its other iPhone apps and integrate more than 200 million listings from eBay into RedLaser's results.
According to eBay's acquisition statement, RedLaser has been downloaded more than 2 million times. It was probably a pretty easy decision for the company to pick it up and integrate it into its suite of mobile applications.