Apple announced Friday that iPhone 3G and iPod Touch users have downloaded more than 500 million apps in about six months, a staggering figure considering it took the iTunes Music Store more than two years to hit that mark.

The pace of adoption also appears to be increasing at a significant rate, as the company said it had 300 million downloads and 10,000 total apps in early December. The number of apps has grown to 15,000 in a little over a month, as developers increasingly see the platform as a lucrative opportunity. While the majority of the downloaded apps are games or entertainment programs, there's a wide array of business programs from the likes of Omniture, Oracle, and Salesforce.com.

Mobile apps have been available for years through places like Handango, but the App Store has seen tremendous success primarily because of the popularity of Apple's touch-screen devices, as well as the company's ability to leverage its iTunes juggernaut.

Since the App Store launched in July, Apple has sold more than 7 million iPhones, and the iPod Touch had a successful holiday season. But the iTunes integration may be what pushed the App Store into the mainstream, as it gave users a simple way to browse, purchase, download, and install apps in a smooth manner.

Apple's rivals are taking note of the success, and they're taking similar approaches to distributing mobile applications. Google has its Android Market, Research In Motion soon will have an online storefront for its BlackBerry smartphones, Palm's webOS will have an over-the-air program to distribute apps, and Microsoft is rumored to be working on its own solution for Windows Mobile.