By Josh Ong

Published: 08:00 PM EST (05:00 PM PST)

Using free sponsored apps, Microsoft has been aggressively promoting Bing search through Apple's App Store.

Microsoft is working with outside developers to raise awareness of its Bing app, according to Nick Wingfield of The Wall Street Journal. In recent weeks, the Redmond, Wash.-based company says, as many as seven apps at once, each with Bing in their names, have held top positions on the iPhone App Store's most downloaded free app lists.

Most of the third-party apps, which are sponsored by Microsoft, allow users to listen to popular music for free, provided they download the Bing app first. The apps have names like "Ryan Seacrest's MixTapes by Bing" and "Bing Hip Hop 100."

According to a person familiar with Microsoft's strategy, the push is meant to "crack the code" of the App Store. Many of the third-party apps are being advertised on mobile advertising networks.

A spokesman for Microsoft acknowledged the campaign and its effectiveness. “It turns out if you have more than one of these apps at the upper-end of the store, you see decent lift across all of them,” Adam Sohn said.

Although Microsoft is currently reworking its mobile phone OS, with Windows Phone 7 due out "before the holidays," the company is still committed to marketing its search product on competing mobile platforms. In addition to courting iPhone users, Microsoft is looking to capture some of the Android search market. An official Bing app for Android is nearing completion.

“It’s all about building a great service and making it as broadly available as possible,” said Sohn.

Earlier this year, rumors arose, but were later retracted, that Apple was in talks with Microsoft to make Bing its default iPhone search service. In June, Apple CEO Steve Jobs announced that iOS 4 was adding support for Bing search, although Google remained the default option.