Apple Inc. is opening its free iPhone and iPod Touch applications to micro-transactions, a move expected to create a bonanza for a wide variety of companies, most notably those selling “virtual goods” and games.

The move will make it far easier to sell upgrades to premium services or subscriptions over users’ phones. It’s also expected to supercharge the small purchases made for free-to-play social-network games from companies like Zynga, Playfish, and Playdom. That’s a market that’s already expected to hit $1 billion this year.

“This market is going to explode,” said Trip Hawkins, CEO of the iPhone-focused game-development firm Digital Chocolate and previously founder of gaming giant Electronic Arts. “This is a capacity that opens up new markets to a lot more consumers.”

Digital Chocolate, a nearly 400-person company headquartered in San Mateo, California, makes games that have been downloaded more than 40 million times in just the past 10 months via Apple’s App Store. The company currently distributes free versions that it uses to entice users to buy paid, premium versions.

“You can see why micro-transactions would probably mean a lot to a company with a business model like ours,” he said. “It could easily double revenue within a year above what it would have been without it.”

Apple, which has sold more than 50 million iPhones, began allowing micro-transactions over paid applications four months ago. The latest change, announced October 16, removes all barriers to making small purchases over the iPhone. Apple already has the customer’s payment information and can simply process transactions, taking a 30 percent cut in the process.

“It’s got implications for all developers and anybody in the virtual-economy space,” said Andrew Schneider, CEO of Live Gamer, a 65-person New York company providing virtual-economy support to gaming companies.

Apple’s iPhone and the iPod Touch have already emerged as major gaming platforms. Since the App Store launched in July 2008, there have been more than 2 billion application downloads, and there are now more than 85,000 free and paid applications, ranging from games to medical monitoring. iPhone users account for 65 percent of global mobile Internet page views, but only 11 percent of the total smartphones shipped, Morgan Stanley managing director Mary Meeker told the Web 2.0 conference in San Francisco this week. Meeker called the iPhone/iPod Touch combination “the fastest growing consumer electronics platform in history.”