Not all of the apps we spotlight here are new or games (though I admit that most of them have been lately). Mint.com's app has been around for a while, and it's just as useful as the site that gives it the name and functionality. If you have a bank account, you should at least give Mint.com a look -- just by plugging in your bank information (which is as secure as it can get on the site), you can start seeing patterns and trends in your spending even without doing anything at all. And after a few months of use, the site will even do things like create a budget automatically for you, tell you when you're spending more than you usually do, and even recommend ways to save your money.
The iPhone app does all of that on the go, giving you instant access to your financial information wherever you are. You can check your accounts, edit your transaction records, and see your monthly budget (so even while out shopping, you can know exactly how much money you've got to spend) as long as your iPhone has a connection. You can get alerts when something goes wrong. And security even on the mobile device is excellent -- there's a password built-in to the app, so anyone using your phone can't get immediate access to your information, and if you lose your phone, you can even disable iPhone access directly from the web app, ensuring that no one who uses your lost phone gets any sensitive information from the app.
Oh yeah -- and it's all free. Mint.com is definitely worth a look if you haven't installed it yet.