With 30 million phones sold, Apple’s (AAPL) iPhone seems unstoppable. And, now that users are permitted to create and sell their own custom-made iPhone applications -- or apps -- a new industry is emerging.

With Apple’s app store hitting 1 billion downloads last month, the variety has been staggering, ranging from the sublime (chess, Wikipedia, and shopping applications) to the ridiculous (a free download called "What’s Your Sex Appeal?"). But big labels are using these apps to change the way brands operate.

“It’s a very new space. If you’re going to be in the digital world, you need to be mobile and if you’re going to be mobile, you need to be on the iPhone,” says Walker Fenton, General Manager of Media and Consumer Business at branded app designer NewsGator, which has simplified the process of creating apps with their proprietary NewsGator iPhone Media App Framework.

“A lot of our clients are media companies: USA Today (GCI), Reuters (TRI), CBS News (VIA), Discovery (DISC). They’re all trying to get their content into people’s pockets. This is an easy way to do it.”

Known as the iFood Assistant, Kraft’s (KFT) popular app offers 7,000 recipes and a grocery store locator, becoming a prime example of what a branded iPhone app can do. Hailed as one of the app world’s most thorough resources, the iFood Assistant has brought the traditional Kraft brand into the digital realm.

But not every popular branded app is so content-rich. The Virtual Zippo Lighter app lights up your iPhone with the flick of a wrist - and that’s it. It’s still too early to obtain real app metrics, but the 125,000 reviews in the app store speak to the popularity of the digital Zippo billboard.

But Fenton sees branded apps shifting towards content-rich media: “There is more content there than ever, new systems where people generate and share content,” he says. “With the iPhone it’s such a dedicated experience and so personal because it’s 12 inches from your face.”

Companies like Nike (NKE) and Hewlett-Packard (HPQ) have followed suit with apps like Nike Goal, which follows Italian soccer scores, and HP’s iPrint Photo, which enables users to print to their HP printer. But the branded-apps realm could ultimately take the digital branding concept away from big labels.

“Ashton Kutcher doesn’t have to be on television anymore. He’s on Twitter and all these other media. All different people are generating content,” says Fenton. “My mom isn’t going to have a branded iPhone app. But there are definitely individuals who have their own app and lots more to come.”