Apple opened its App Store Thursday with more than 550 applications for the iPhone and iPod touch, about a fourth of them free.

Available via iTunes, which was updated earlier Thursday to version 7.7, the App Store is Apple's online mart for downloadable iPhone and iPod touch applications.

Among the first wave applications are AOL's AIM instant messaging client; Apple's own Remote, which lets users control iTunes on a Mac or PC using an iPhone or iPod touch; and the location-based Where. Of the 552 applications currently showing in the store, 134 can be downloaded free-of-charge; the most expensive, My Life Record, a personal medical records system, sells for US$49.99.

As of 11am US EDT Thursday, four of the five most-popular applications on App Store were games: Super Monkey Ball, Cro-Mag Rally, Enigmo and Texas Hold'em. In the second spot, however, is MLB.com At Bat, which delivers real-time scores and video highlights of major league games.

Although the App Store is live in iTunes, it's currently hard to find from the latter's home page. This link, however, leads users to all iPhone applications; from there, click on "App Store" in the bar immediately above the display area.

The strong debut of App Store isn't surprising, said Ezra Gottheil, an analyst at Technology Business Research Inc. "Everybody is building something for this thing," said Gottheil. "This is a very rapidly growing platform and every developer and his brother is writing code. Plus, Apple is giving them a [distribution] channel. Some of them may be complaining about the 30 per cent, but 30 per cent to cover all the marketing is a great deal."

Apple, which introduced the App Store in March, lets developers set the price for their wares, or if they want, give them away, but takes 30 per cent of the revenues for providing the marketing muscle and App Store bandwidth. A program's developer then keeps the remaining 70 per cent of any income.

Although the App Store is currently available from iTunes 7.7, it will also be accessible directly from an iPhone or iPod touch when those devices' firmware is upgraded to version 2.0. That upgrade, which will also add Exchange synchronization features to the iPhone, will be released later today or early Friday, in time for the roll-out of the new iPhone 3G. Some users, in fact, have reported that they were able to grab the 2.0 update from Apple's servers.

The 2.0 upgrade will be free to iPhone owners, but iPod touch users must fork over US$9.95 for the update.