I've been using Mint.com (not to be confused with Shawn Inman's great Mint web stats tracker) to track my financial information for a while now. It's very nice -- completely free, and hooks right into your bank accounts, investments, or other financial accounts online (securely, of course -- they've got over 7,500 institutions in the system) to let you know what you're spending, when, and where. I'm lazy when it comes to budgets, but Mint will automatically make budgets for you based on what you spent the month before, so I can see if my groceries, bus fare, or, ahem, iPhone app budgets go over one month.
And now they're released an iPhone app, so you can have access to all of that information on the go. It's pretty much a portable Mint -- you've got all of your usual account information, alerts are pushed off onto their own screen, so you can see at a glance where you're overspending, and you can flip through and see your expenses for the month, where your cash is going, and any budgets you've programmed into Mint. The app is quick and easy to use -- like Mint.com, but in the palm of your hand. It's free and on the App Store right now (though of course you have to set yourself up on their website first).
One security issue: the app wisely recommends that if you don't have a passcode on your phone already, you put one on it, since your financial information will be available to anyone using your phone. The Mint.com folks also tell us that you can disable iPhone app access from their website, so even if you do lose your phone, you can keep unwanted users from reaching your information. But as with everything, there's a tradeoff of convenience and security -- while it's helpful to have this information anywhere you are, the flip side is that it's that much more likely to fall into the wrong hands.