How much? Oh, a few bucks here, a few bucks there. But it all adds up. Take GasBuddy, the find-the-cheapest-gas app. If you fill your gas tank twice a week and you're able to save $2 every time, that's $16 per month -- and $192 per year. Not too shabby, eh?
Cellfire, MobileCoupons, and Yowza These apps, all of which are free, will find and display coupons for businesses in your area. In a matter of seconds you might be able to save 10 percent on your lunch bill or get a buck off your next mocha latte. Different stores push coupons through different apps, so I recommend stocking up on several of them.
GasBuddy Saving a nickel or dime per gallon of gas may not seem like much, but it adds up over time. Gas Buddy ($2.99) shows you which nearby stations have the lowest prices (and provides maps if you don't know the area), so you can pocket that extra change every time you fill up. (Obviously this app works best for those who live in densely populated areas--and drive a lot.)
Inrix Traffic Time stuck in traffic is time you're not making money. This free app provides real-time traffic maps so you can avoid trouble spots and, hopefully, get where you're going a lot quicker. And if you're already stuck, it can estimate how much longer the delay will be -- so you know whether it's worth getting off at the next exit or waiting out the traffic.
MotionX GPS Drive Instead of dropping $100 on a standalone GPS or paying AT&T $10 per month for its navigation service, try MotionX's app. When you buy it for 99 cents, you get 30 days of real-time routing. After that, it'll cost you just $2.99 per month or $24.99 annually. But you don't have to subscribe: You can pay as you go, dropping that three bucks only when you need "live" navigation.
Pic2shop So you're in the bookstore, about to pick up a copy of "The New Rules of Marketing and PR, 2nd Edition," when it occurs to you that the book might be cheaper online. Fire up Pic2shop (free), point your iPhone's camera at the book's barcode, and in seconds you'll see online-price comparisons. It works with countless other products as well.
Undercover Ever stop to think what a lost or stolen iPhone would cost to replace? It's not pretty. Undercover can help you recover a missing iPhone by tracking its location and remotely generating a pop-up message. Not bad for an app that costs $5, with no subscription fees. You can read my full coverage of Undercover over at iPhone Atlas.
Know of any other money-saving apps? Hit the comments and spread the word.