According to estimates from BusinessWeek, Apple is poised to hold a nicely sized 21% of the mobile ad market share by the end of the year. If this estimate turns out accurate, Apple would be right up there with Google as one of the top two players in this space. While Google purchased AdMob as part of their initiative to own the mobile ad marketplace, Apple’s somewhat recently announced iAd platform has caught steam quickly.

Microsoft has opened their own version of Apple’s App Store. These “Apps,” which can be downloaded from the “Marketplace,” are all free, professionally designed and coded by Microsoft for their store, and available for their new Zune HD media device. However, free is the key word in this phrase. Microsoft has decided to use pre-roll ads each time an application is launched to get their money out of them.